WHEN the spectrum is allowed to fall on a sensitive plate we can, as has been mentioned, obtain a photograph of it, but, unless special means are used. not of all the lines. The photograph obtained with the salts of silver will fail altogether to reproduce the yellow part; will show something of the green and nearly all of the blue; while up in the violet end the picture is very clear, and beyond the violet, where to all appearance the spectrum has ended, a host of sharply-defined lines comes out on the plate from a region where the keenest eye sees nothing whatever. This is when the instrument is directed full on the sun (not necessarily on its edge, as in a former experiment), and it would appear at first as if there must be in the white sunlight a special kind of rays, which produced not colors or vision, but chemical changes on the plate, printing there images of the slit, which were produced by something quite different from light. If, on tbe other hand, we take a delicate thermometer or II radiometer, and move it into successive parts of the spectrum formed by a prism, we find little effect in the blue, more in the yellow, still more in the orange, and as much or more quite beyond the red. wbere, too, the eye sees nothing. Again, it seems at first that here is another kind still of radiation, causing heat, and which is distinct from that producing light, since one appears where the other does not. In some text books yet in use, diagrams even are given to show the amount of chemical, light, and heat rays in the different parts of the spectrum; but quite recently students of science arrived at a better understanding. The results of old and modern investigations are now seen to point to one conclusion. Given in general terms, this may be said to be that there is, in reality, no such thing as a chemical ray. a light ray, or a heat ray; there is nothing but radiant energy—motion of some kind, causing vibrations across space of something between us and the sun—something which, without understanding fully, we call” ether,” and which exists everywhere, even in the “ vacuum '' of a radiometer. These vibrations are measurable with great accuracy (by processes of which an explanation would be here out of place), and are found to be extremely small in all cases, but to vary among themselves, somewhat as those coarser ones do which have been long known to produce sound. As the high notes of a piano are caused by the rapid vibration of strings, and the low notes by comparatively slow ones, but the sound, whether acute or grave, is due to one thing—motion of the air; so the miscalled “ chemical” or “actinic” rays, as well as those which the eye sees as blue, or green, or red, and those which the thermometer feels, are alfdue to one thing—motion of the ether. Rapid motions exist, which set the molecules of silver vibrating, and are registered by the photograph. These fall also on the eve and on the thermometer d ee. ments are fitted to register, The longer radiations in turn carefully covered with lampblack to enable it to absorb as many radiations as possible, and the inside of the sphere is sa inclosing a small thermometer at its center, t. The bulb is co ia blackened in the same way. Suppose the temperature of the whole at first to be that of absolute cold or at the natural zero, and that the sphere is kept at that, whatever happens. It' we remove a given part of the sphere,.let us say one-twentieth of the surface area, A B, and fill the aperture with a piece of white-hot iron, this will send heat to t, and the thermometer will rise, though not to the temperature of the iron, which, for the sake of illustration, we will call 2,400°. If the whole sphere were at 2,400° the thermometer would also shortly register this (provided we could make one to stand it), but in fact it is receiving such heat from one-twentieth of the sphere only, and giving it effect larger receptacle for the same purpose as those at A, only of a size to accommodate such pictures as are mounted in wood, as many lantern slides are. To produce the intended effects in endless variety, a picture—say of statuary—is inserted in the first slot at A. Now examine its lovely appearance, and tben insert in the next slot, behind it, a view of an interior of a cathedral or a picture gallery, and you at once find the statuary piece to have new beauty and interest, for it is located properly in some harmonious interior. The effect of the whole is now heightened by placing a blue, or red. or golden tint in the groove next behind, or at C. Or the first picture may be a view with splendid foreground and empty sky. Natural clouds may be made to treble the effect by placing a cloud transparency behind the other, and the tinter may then be used or not, as you please. Or it may be you have a beautiful waterfall in first which you can variously tint, and then from its foaming face bring out a splendid statue, with effect almost equal to that of a dissolving lantern. Sunset, sunrise, and moonlight effects may be added to properly chosen landscapes, and day turned into night. and vice versa. The majority of the effects obtainable with a lantern may be secured with the touroscope, and the number of changes possible with it are only limited by the genius of the one working it and the quantity of pictures at hand to work with. To those who have a variety of transparencies it will be particularly welcome, for by its means they can be used at day or night, and in any light, just as effectively, and as well singly as in combination. If the portrait photographer would have one on his recep-tionroom table, with a few portrait and other transparencies, he could, by properly pushing the thing, sell many transparencies from his negatives, and touroscopes to exhibit them in. Here is real business for those who want it. Undoubtedly the touroscope. if taken hold of, will open up a new avenue for the growth of photography, and create a demand for a new style of picture. Neither the stereoscope nor the graphoscope was much thought of at first, but see what wonderfully profitable things they have been for our art.—PM. Plwt. bulb or radiometer, and produce some kind of mechanical ~ in a minute degree. but not one which those instru-are fitted to register, The longer radiations in turn are not themselves “ heat,” any more than those which the retina of the eye responds to and calls “light.” We have always one and the same cause—radiant energy; and we give this one thing different names: “actinism, “ “light, “ or Fig. 27.—SECTION OF CALORIMETER. "heat,” according as the instrument which reveals its presence to the mind is some chemical substance, the retina of the eye, or a thermometer. It will appear from what has just been said, that there are substances which respond to some of the ethereal vibrations and not to others. The substance which is most generally useful in receiving and. so to speak, absorbing them, is perhaps that which has been recently put to such remarkable use by Edison—common lampblack. Let us try to measure the sun's radiant energy by measuring all of it we can get in the form of heat. and endeavor in the process to reach some idea of the temperature of its surface. There are many ways of measuring the heat, one of which, convenient for its exposition of principles, we give here, though it is not perhaps the best in practice, returning to other methods later. Thu3, in Fig. 26, let A B DEbe a large hollow sphere, Fig. 28.—CALORIMETER. out by reradiation from the bulb to the other nineteen-twentieths, that is, to the whole cold surface around it, which returns nothing. In this case, then, the temperature of the thermometer will be found by reflecting that it gives out very nearly twenty times as much heat as it receives, and that it must register nearly Af$-°, or 120°. On the other hand, suppose we, in a neW experiment, find the thermometer reads 100°, and want to know the temperature of the iron. We must find what proportion the hole, covered by the hot iron, bears to the whole sphere, and multiply the 100° by this. Were the hole, for instance, m this case but one-thirtieth the size of the sphere. evidently the temperature of the hot iron must been about 3,000° . If the iron were ever so distant, provided it filled the whole aperture to an eye placed where the bulb is, no external rays could fall on t except from it. It is immaterial, then, in this experiment, whether the hot body is near or far, provided the hole is always kept so small that no foreign radiation enters. The reader will see the bearing of this when he reflects that if we turn the opening II tor- sphere toward the sun, with the above precautions, the result will be just the fame as if we had plugged the aperture wit h a simple piece out of the sun's photosphere and of its actual temperature. We have now only to multiply the thermometer reading by the number of times the surface of the sphere is greater than the hole, and we have apparently found the real temperature there, as exactly as if we had reached across space and dipped our thermometer bulb into the actual surface of the sun. There are many drawbacks to this plan in practice, and it is only II case radiation and temperature are proportional that it is sound m theory. Variously modified, however, it is much relied on by experimenters. Fig. 27 gives an internal, and Fig. 28 an external view of the latest construction adopted by M. Violle, of Grenoble, a distinguished recent investigator. In practice the simplicity of our first illustration is widely departed from, and the use of the instrument is much modified. -T is the thermometer, whose bulb is at the center of a double sphere maintained at 0° (Centigrade) by a current of ice water circulating through tubes, t t', or by ice put in at O. D is a diaphragm with various apertures; M, a mirror, in which we view the reflected image of g/ g is Fij.29. i' A' ACTION OF LENS. a piece of ground glass, on which the shadow of the thermometer bulb falls when the instrument is correctly pointed to the sun. This instrument is capable of being used to give us (according to the method just explained) the temperature of the sun, or else the number of units of heat it sends out. The latter result will be presented, however, by another method subsequently, but before we can do either accurately we must find out how much heat is absorbed by our air. To do this. M. Violle has taken his whole apparatus to the summit of Mont Blanc, and finds there the radiant heat from the sun to that below almost exactly as 4 to 3. The total heat at the boundary of our atmosphere is, according to him, something like one-half greater than at the sea level, a rather larger result than one obtained by another means, to be given later. To find the temperature of the sun from such an apparatus we virtually multiply the thermometer reading by the fraction expressing the ratio of the surface of the sun's disk to that of the celestial sphere. a ratio which is rather less than 1 to 180,000. In the observations of Soret, on Mont Blanc, the inclosed thermometer read nearly 38° Fah. above the temperature of the inclosure, and hence the temperature of the sun's surface would appear to reach at least the enormous number of 38°X180,O00=6,840,000° Fah. The more prolonged and elaborate experiments of Mr. Ericsson give a temperature of about 4,000,000° Fah., and indicate that each square foot of the solar surface radiates over 300,000 units of heat per minute; in other words, each foot can furnish heat equal to that required to drive a theoretically perfect heat engine of over 7,000 horse power. There is a very fair agreement among all experimenters as to the amount of heat radiated, but a wide discrepancy as to the temperature, the very same data which above are interpreted as meaning 4,000,000° Fah. being asserted by distinguished Frenchphy-sicists to indicate less than 4,000° Fah. This monstrous disagreement is not due to any considerable error of measurement—all are pretty well agreed on that—but to our ignorance of the laws connecting temperature and radiation. There are two rules in use, one of which was given by Sir Isaac Newton. It says, in substance, that if a body be raised to double its former temperature, it will radiate double its former heat. The other, given by the French physicists Dulong and Petit, is in the shape of a complex formula, which virtually declares that if a body be raised to double its former temperature it will radiate more than double its former heat; in case both temperatures are high, enormously more. Proving that we get enormous heat from a limited area of the sun's surface, then, does not, in the eyes of some physicists, prove that area to be proportionately hot. In this there is involved a very practical consideration for us, for this apparently abstruse physical question has a bearing on the duration of the human race, since that duration depends not merely on the present heat of the sun. but largely on the rate at which the sun is spending heat. Suppose some benumbed wanderer to find himself before a fire which seems as if miraculously burning for him, in a cheerless waste, where he would otherwise perish. A fire of straw may be for the moment as hot as a fire of coal; but as the first will spend its stock of heat at once and leave him to die of cold, and the second will spend it slowly and warm him for indefinite time, it is an important thing for him to know therate atwhich his fire burns, and this is our own case. The human race—however it came here—finds itself before such a fire, and thus dependent upon it; for it lives on a planet whose proper surface temperature in the absence of solar radiation is variously estimated at from 70° to 273° below zero; and we are all warming ourselves at the sun, without which we should promptly die. D'fi30 j SECTION OF A POLYZONAL BURNING LENS. Let us come bank to the question of the sun's temperature then, with a sense of its personal interest to us. We should know more about it if we could carry our thermometer nearer to the sun, but we can practically do so by means of a burning lens, Fig. 29, where S F S' is the real angle subtended by the sun, 8 F i' that which it is made to appear to subtend by the lens, so that the effect is nearly that which would be produced by approaching till the solar diameter filled S S'. The actual construction of the burning glass on a very large scale is not now common, as we have other ways of producing intense heat always at command. When made at present they are built up in sections, as in Fig. 30, so as to avoid the necessity of an enormously thick and expensive lens. Such a one as this, in which the lens subtends an angle of about 30°, as seen from the focus, is capable of melting platinum and the most refractory surfaces; and as a great deal of the heat is absorbed by the glass' or otherwise lost, if we could approach the sun till it filled such an angle to the eye, we should find the temperature even higher. It is probable that few of the materials of which the crust of the earth is composed would remain in the solid form if carried very much nearer the sun than the presumed orbit of the hypothetical “ Vulcan;” and it may be remarked in passing that it is not unlikely that, in case such an intra-mercurial planet as Professor Watson is said to have recently discovered had an orbit whose nearest approach carried it within 10,000,003 or 12,000,000 miles of the solar surface, it would prove to be heated to the point where it would be self luminous. The writer, some time since, made a comparison of the light of the sun with that given from the molten steel in the Bessemer converter. This was chosen as an example of the greatest temperature attained on the large scale in the arts, and it is one which is known to equal that at which platina melts. Looking down the mouth of the converter we see at one stage of the process a stream of molten iron poured into the vessel in which the melted steel is already glowing in the background. Every one knows how bright white hot (and still more melting) iron appears, but in this case the steel is so much brighter, that the fluid iron in front seems like thick chocolate poured into a white cup. The steel, just before it is itself poured, seems of sun-like brilliancy, until we come to compare it with the sun itself, which was done by means of a photometer, so arranged that the steel light shone in at one side and the sunlight on the other. When every precaution showed that any single square foot of the solar surface must be giving out much more, at any rate, than one thousand times the light that the melted steel did. We are not, it is true, entitled to conclude from this that the heat is in exactly the same proportion, but we are justified by inference from this, and by other experiments not here given, in saying not only that the temperature on the sun's surface is far higher than that reached in our furnaces, but that the heat is in fact so enormously greater than any furnace heat here that they can scarcely be made the subjects of comparison. Other considerations, on which we cannot now enter, give the best grounds for belief that this heat is likely to he kept up sensibly at its present rate of emission for a period which, with reference to the brief history of the human race, may be called almost infinite. These are important conclusions, whose practical bearing will be more fully developed in what follows. When we watch a gentle summer rain, does it ever occur to us that this familiar sight involves the previous expenditure of almost incredible quantities of energy, or do we think of a drizzly day as perhaps calling for a greater exertion of Nature's power than an earthquake? Probably not; but these suppositions are both reasonable. the angle subtended by each source of light was equal, the image of the molten steel was put out by the presence even of much enfeebled sunshine, and ceased to be visible, as the dull flame of an alcohol lamp would be if it were set beside an electric light. The area of glowing metal exposed was considerably over one square foot, an a measures made with THE PYRHELIOMETER. Take Manhattan Island, for instance, which contains 20 square miles, and on which one year with another over 20 inches of rain falls. (To be within the mark we will call the area 20 miles, and the annual rainfall 30 inches.) One square mile contains 640 acres, and each acre 43,560 square feet. Multiplying by 640 and dividing by 12 we have 2,323,200 as the number of cubic feet of water on 1 mile in a rainfall of 1 inch, and as a cubic foot of water weighs 997T1O'0'5 oz. avoirdupois, and there are 35,840 oz. to the ton, this weighs 2,323,200 X 997 137 . J ,.., -, or, in round numbers, 64,636 tons (to 1 35,840 mile and 1 inch of rain). As there are 20 miles and 30 inches, the annual rainfall on this little island is 1,393,920,000 cubic feet, or 38,781,600 tons. The amount of this may be better appreciated by comparison. Thus, the pyramid of Cheops contains less than 100,000,000 cubic feet and weighs less than 7,000,000 tons, and this water, then, in the form of ice. would many times replace the largest pyramids of Egypt. If we had to cart it away, it would require 2,231,800 cars, carrying 12 tons each to remove it, and these, at an average inch of rain spread over the whole area of the United States is not an extraordinary day's rainfall throughout its territory, but it will be found by any one who wishes to make the computation that such a day's rain represents a good deal over the round sum of ten thousand of millions of tons, and that all the pumping engines which supply Philadelphia, Chicago, and our other large cities, dependent more or less on steam for their water supply, working day and night for a century, would not put it back to the height to which it was raised by the sun before it fell. Every ton was lifted by the silent working solar engine, at the expense of a fixed amount of heat, as clearly as in the case of any steam pump, and this is the result of an almost infinitesimal fraction of the heat daily poured out from the sun! Now heat is something men have only in quite modern times learned to think of as a measurable quantity, and we must remember that we cannot even begin to have accurate knowledge of any form of force till we can answer the question, “how much about it, not vaguely, but in figures. When we hold the right hand in warm water, the other in cold, for a few moments, and then plunge both in the same basin of tepid water, the two hands will give different reports; to the right the fluid is cold, to the left it will feel warm, though it is the same really to both, and we might vary the experiment by trying it with shade and sunshine. In either case the experiment would convince us that our sensations were very untrustworthy, and that if we were going to measure the sun's heat we must depend on some sort of instrument and not on anything that can feel. The first things we have to do about the sun's heat is to measure it, not to guess at it—to measure it as accurately as we would anything which we could try with a foot rule or put in a pair of scales. When we have done this we have a solid foundation to work on, and the doing this has been thought a worthy occupation of a considerable part of their lives by many able men. One of the first of these was Pouillet; others, such as Saussure and Herschel, had been at the problem before him, but his results were the most accurate until very recently, and even recent work has not materiallyaffected his conclusions. His instrument is easily understood with a little attention. We have it represented in Fig. 31. Let lis first remark, that what we want to get is the sun's direct or radiant heat, quite irrespective of that of the atmosphere around us, and that to get definite results. by our present method, we want to know how much of this radiant heat falls on a given surface of one square foot or yard. We may reckon it by any one of the numerous effects heat produces; practically it is convenient to let it warm water, and to see how much it heats, through how many degrees, and in how many minutes. Pouillet's pyrheliometer is substantially nothing but a very shallow cylindrical box, A A', filled with a measured quantity of water. It is mounted on the end of a hollow rod, having at its other extremity a metal disk of the same size as the water box. When the shadow of the box exactly covers M. ERICSSON'S SOLAR CALORIC ENGINE. length of 30 feet to the car, would make 6 trains, each reaching in one continuous line of cars across the continent, so that the leading locomotive of each train would be at San Francisco before the rear had left New York—a result which appears at first so incredible that it seems best to give the figures on which we rest the statement. Now this is for a very small part of a single year's work of the sun in raising water to produce rain on the little spot of Manhattan Island alone—a spot, geographically speaking, hardly visible on the map of the country. Again, lr of an the disk the ind nanent is pointed true on the sun. Held in the hollow rod is an inverted thermometer, whose bulb is within the water box. A A'. This enables us to read the temperature of the water from moment to moment. It is not enough to expose it for a time to the sun and read the thermometer—this would give too small a result, because the instrument as soon as it is warmed commences to radiate the heat away again, like any other hot body; and we would like, if we could, to keep all this heat in it to measure. As we cannot, we reach the same result by finding how much is ?9999999999999? 82 lost, and allowing for it. Thus, the observer first leaves the apparatus in the shade (for instance) five minutes, and notices whether it loses or gains from its own radiation to surrounding objects, Then he leaves it directed to the sun, which shines full on it for five minutes more, the thermometer be- j ing read at the end of this exposure; and finally, at the end j of another five minutes, during which the instrument has . been left in the shade, it is read again, The half sum of the losses or gains in the shade is the radiation, and this added to or su btract ed from the apparent gain in the sunshine is ' the actual number of degrees that the temperature of the ' water would have been raised, had all the solar heat been retained, Measuring in this way, we are independent of the temperature of surrounding objects. j Mr. Ericsson, the celebrated engineer, who has improved ' on Pouillet's apparatus, has in fact shown that we do in accurate experimenting always get more heat (other things being equal) on a day in winter than in summer, as we should, if it is the direct solar radiation alone we are after; for that will be the greatest when the sun is nearest, as it is in our northern winter. Again, measuring when the sun is high, and at all altitud es down to the horizon, we find less and less heat, as the rays go through more of our atmosphere, and hence we can make a table showing how much this absorbs for every al ti tude, ami consequently how much we should gain if it were taken away altogether, When this is done we find, according to Mr. Eri csson's late determinations (which we substitute for Pouillet's), that the direct heat of the sun on 1 square foot in March is competent to raise 7'11 pounds of water 1 ' Fah, in one minute, This is what it would do if we got outside of our atmosphere; but owing to the absorbing action of this, the radiation which actually reaches us, under a vertical sun, will so heat only about 5 6 pounds, According to the mechanical theory of heat this effect is that which would be required to drive an engine of 5'6 X 772 ' —tjtt.vk-,— = 0 '131 horse power. In other words, the heat of j a vertical sun after absorption by our atmosphere represents rather over one horse power to each square yard. It is true that we cannot always have a vertical sun, nor clear sky, nor can we realize in actual work this whole effect by any form of but when we have made the largest deductions the statement of the sun's power, in this form, is calculated to exci te astonishment, VVe have here, since there are 5, “80 linear feet in a mile, 5380° X 0131 = 3,650, 000 horse pow er to the square mile (in round numbers) ; so that if we suppose in actual practice one horse power realizable to square yards, the efficient working power of sunl ight on an area much smaller than such a region, for as the Adirondacks, is much greater than that of the computed actual steam power of the whole world. Upon the su face of the whole earth the heat at any time must be equal to that falling vertically on one of its great circles, which contain, rou ghly, about 50,000,000 squ are miles, Here, when we come to multiply the number of miles by the power per mile, we reach figures bew ilde ring in t heir magnitude, but which are demonstrably correct, The only way this heat is utilized by conversion into power at present (steam power being dependent on coal made by the sun in past times) is by windmills and waterwheels, both supplied by the sun, as in fact in every form of power, unless we except the in significant one of tide mills, a kind only ill a very remote degree dependent on solar action favoring circumstances, he actually realizes about 10 calories per minute per meter, which is a trifle less than one horse power to ten feet square, Something exceeding this might probably be reached with the same apparatus in a drier air; upon the whole we are justified in speaking of one horse power to the square of ten feet on the side, as actually realized; one horse power to one square yard being about tbe limit of that which is theoreti call y realizable. It must be remembered that, according to what has been stated, the sun offers us a source of power which is prac-tjcally infinite both in amount and duration. According to what we believe we know with assurance, we can say that the sun is not a fire, fed by any fuel, but a glowing gas ball, maintained at an enormous temperature, and radiating enormous heat from a fund of energy maintained by the contraction of its volume, and by the impact of meteoric bodies, We can reckon with confidence that there will be no material diminution of its supply from these sources for a duration only to he reckoned by hundreds of thousands of years. As to the amount of heat supplied, it is i nconceivable, The writer has made a computation of the time all the coal of the world would suffice to main-tain the sun's radiation, were the actual source of it to fail, and were our whole supply of coal transported to its surface and burned there in its place. The result, otherwise stated, is that in any one second the sun radiates into space an amount greater than could be made good by totally consuming all the known coal beds of the world! SomethII g like 300 years separates the England of to-day, w ith her countless furnaces and engines, from the England of Elizabeth, in whose reign the spinning wheel was almost the most intricate piece of machinery on the island. Some-thin g like 300 years more, it is said, is all that separates the England of to-day from the future England whose furnace fires will h a ve died out with the flame of th e last bushel of, coal under her surface; whose harbors send out only sailing I craft; whose manufacturing population has gone to other i lands, and wh ose “ black cOuntry” is grow ing green again as nature covers the ashes of her burnt-out mineral wealth with new verdure for the few who remain on tlie soil. We do not pretend ourselves to join II such pessimist views, or try to look into the future so far, though th is is a very little way com pa red with what we know of the rise of man to civilization. To us, II this country, such a time, if it is ever to come, is immensely distant. But wh at is certain is t hat if some such change do not take place it will be th rough the discovery of a new source of power, for of the old, the coal, when our undelground supply is used up we cannot get i any more, Let us remember, then, II time, that though the i stock be great there is no renewal. , For a journal counting among its readers so many inter-1 ested in the applications of power as the Scientific American, I have thought, elementary as this presentation of the sun's claim to interest,, merely as a source of mechanical ! power, is, it is better to offer it, We are, in closing, led back ,' to the suggestion with which these articles began, of the sun 's influence in altering the conditions of existence for the ' human race. j Future ages, it has been truly enough observed, may see | the seat of empire transferred to regions of the earth now I barren and desolated u nder in tense solar heat—countries J winch, for that very cause, will not improbably become the 1 se.it of mechanical and thence of political power, Who, . ever finds the way to make industrially useful the vast sun power now wasted on the deserts of North Africa, or the j sh ore s of the Red Sea, will effect a greater ch ange in men's affairs than any conqueror in history has dune, for he will j once more people those waste places with the life that I swarmed there in the best days of Carthage and of old | Egypt, but under another civilization, where man no longer worships the sun as his god, but has learned to make it his servant.
This article was originally published with the title "The Sun's Radiant Energy"