It is not difficult to imagine that if a small stream of water descending from a hill side were directed into the mouths of the earthern vessels or wooden buckets of wheels used for irrigation, the vessels so loaded would descend and the wheels revolve, so that rotary motion and mechanical power would be gained; the buckets emptying themselves at the lowest point, as they wore before emptied at the highest; the wheel turning in the opposite direction, because the weight or gravity of the water was now the moving power of this overshot wheel. In the undershot wheel the impulse of the water striking the floats drives the wheels; i n the overshot wheel the' weight of the water flowing into the buckets turns the wheel, and all impulse must be avoided; the water must flow vrith the same velocity as the wheel, or just so much in excess as will prevent the buckets from striking the water as they present themselves to be filled. Experience soon showed that the earthern j ar or the suspended bucket were cumbrous and inconvenient, and as larger and more powerful wheels were applied to more copious streams, a series of simple wooden troughs formed across the face of the wheel were found to answer the purpose better. When the supply of water was ample and the wheels large, it was found that to fill these troughs well and regularly the stream should be made nearly as broad as the wheel, and shallow in proportion to its width. The wheel was then formed by placing two sets of arms, at a sufficient distance apart, upon the axle, and fixing to their ends segments of wood to form the circle; upon these segments across the face of the wheel, and equal to, or somewhat exceeding in length the width of the stream or sheet of water, were nailed the sole-boards; on the end of theso boards, and at right angles to them, so as to forma projecting rim or ledge on each pide of the wheel's face, was fixed the shrouding, formed of stout plank generally from 12 to 18 inches broad; and between these shroudings, across the fac of the wheel, were placed the buckets, made of lighter planking, and having their ends let into the shrouding, by which the ends were closed. The edge of the bucket board meeting the sole plank formed two sides of a triangular trough, the third being open to receive the discharge of water. Subsequently the bucket was made in two boards, one called the front, and the other the bottom of tho bucket, the latter taking off the angle and making the section of the bucket, or form of the trough, that of a trapezium, which form it long retained, until the buckets of water wheels were made of iron plate. Since water wheels have been made wholly of iron, and chiefly of wrought iron, the form of the bucket has been either a part of a circle, a cycloid, an epicycloid, or an Archi-median spiral. These forms are noticed in a subsequent page in connection with breast wheels. Great pains are Jiow taken by the best mak'irs of water wheels to form and adapt the curve of the buckets so that they may readily fill with water, retain their load as long as possible, and discharge it with facility when it has ceased to be useful. Mr. Smeaton had the merit of proving and demonstrating the advantage and the diflerence of efiect resulting from employing the weight instead of the impulse of a volume of water descending from a given hight. In reasoning without experiment, one might be led to imagine that, however different the mode of application is, yet that wherever the same quantity of water descends through the same perpendicular space the natural effective power would be equal; supposiqg the machinery free from friction, equally calculated to receive the full eflFect of the power, and to make the most of it : for if we suppose the hight of a column of water to be 30 inches and resting upon a base or aperture of 1 inch square, every cubic inch of water that departs therefrom will acquire the same velocity or momentum, from the uniform pressure of 30 inches above it, that 1 cubic inch let fall from the top will acquire in falling down to the level of the aperture; one would therefore suppose that a cubic inch of water let fall through a space of 30 inches, and then impinging upon another body, would be capable of producing an equal efiect by collision, as if the same cubic inch had descended through the same space with a slower motion, and produced its effects gradually; for in both cases gravity acts upon an equal quantity of matter, through an equal space; and, consequently, that whatever was the ratio, between power and efiect in undershot wheels, the same would obtain in overshot, and indeed in all others; yet, however conclusive this reasoning may seem, it appears upon trial, that the effect of the gravity of descending bodies is very different from the efiect of the stroke of such as are non-elastic, though generated by an equal mechanical power. Gravity, it is true, acts for a longer space of time upon tho body that descends slowly, than upon one that falls quickly : but this cannot occasion the difference in the effect; for an elastic body falling through the same space in the same time will, by collision upon another elastic body, rebound nearly to the hight from which it fell : or, by communicating its motion, cause an equal one to ascend to the same hight. The observations and deductions which Mr. Smeaton made from his experiments were as follows : First. As to the ratio between the power and effect of overshot wheels. The effective power of water must be reckoned upon the whole descent; because it must be raised to that hight, in order to be in a condition for producing the same efiect a second time. The ratio between the powers so estimated, and the efiect at the maximun as deduced from the several sets of experiments, is shown to range from 10 to 7-6 to that of 10 to 5*2; that is nearly from 4 to 3, and from 4 to 3. In these experiments, where the heads of water and quantities expended are least, the proportion is nearly as 4 to 3; but where the heads and quantities are greatest, it approaches nearer to that of 4 to 2, and by a medium of the whole the ratio is that of 3 to 3 nearly. We have seen before, in our observations Upon tha efiects of undershot wheels, that tho general ratio of tho power to the efiect when greatest was 3 to 1; the effect, therefore, of overshot wheels, under the same circumstances of quantity and fall, is, at a medium, double to that of the undershot. Second. As to the proper hight of the wheel in proportion to the whole descent. It has been observed that the effect of the same quantity of 51 water descending through the samespace is double,when acting by its gravity upon an overshot wheel, to what the same produces when acting by its impulse upon an undershot. Therefore the whole hight at the fall should be made available, because, when the water is laid upon the top of the wheel, it is upon the gravity, and not the impulse, that the effect depends. A suflBcient fall, however, must be given to lay on the water with a velocity somewhat greater than that of the circumference of the wheel, otherwise the wheel will not only be retarded by the buckets striking the water, but a part of it will be dashed over and lost, while the buckets will not be so well filled : but no greater velocity should be given than is sufficient to accomplish these objects, as it would be power wasted. Third. As to the best velocity of the wheel's circumference in order to produce the greatest effect. If a heavy body fall fairly from the top to the bottom of the descent, it will take a certain time in falling, but during the fall no mechanical effect is produced; for in this case the whole action of gravity is spent in giving the body a certain velocity; but if this body in falling be made to act upon something else, so as to produce a mechanical effect, the falling body will be retarded, because a part of the action of gravity is then spent in producing the eff ect, and the remainder only in giving motion to the falling body; and therefore the slower a body descends, the greater will be the action of gravity applicable to produce a mechanical effect. If an overshot wheel had no friction, or other resistance, the greatest velocity it could attain would be half a revolution in the same time that a heavy body laid upon the top of it would take to fall through its diameter, but no mechanical effect could be derived from the wheel. It is an advantage in practice that the velocity of the wheel should not be diminished further than what will procure some adequate benefit in point of power, because, as the motion becomes slower, the buckets must be made larger, and the wheel being loaded with water, the stress upon every part of the work will be increased in proportion. Mr. Smeaton's experiments Showed that the best effect was obtained when the velocity of the wheel's circumference was a little more than 3 feet in a second; and hence, it became a general rule to make the speed of the overshot water-wheels at their circumference 3 feet per second, or 210 feet per minute. Experience showed this velocity to be applicable to the highest water wheels as well as the lowest, and if all other parts of the work be properly adapted thereto, it will produce very nearly the greatest effect possible; but it has also been practically shown that the velocity of high wheels may be increased beyond this rate without appreciable loss, as the hight of the fall and the diameter of the wheel increase, and that a wheel of 24 feet high may move at the rate of 6 feet per second without any considerable loss of power. The author has constructed several overshot water wheels of iron 30 feet diameter and upward; and for these he has adopted a speed of six feet per second with great advantage. Fortland Cement and Tar for Roofing. Reid's Treatise on Portland Cement contains the following directions for making roofs of that material in combination with tar. 1st. The inclination of the framework of the roof (which must have an even surface) should be at the rate of from one half to three quarters of an inch per foot. The rafters or joists should not be more than 3 ft. 3 in. apart, so as to give sufficient strength. As the rafters rest on the side walls, a comparatively small quantity of timber is required. Boards of an inch or an inch and a quarter thick, are fastened or nailed on the rafters, and should be dovetailed. These are then covered with a layer of sand a quarter or half an inch thick, in order to produce an even surface. 3d. Strong brown paper, in continuous rolls, and as broad as possible, is then laid upon it, so that each length overlaps the other by about four inches. When the whole, or a large part his thus been covered with paper, the mixture is put into a cauldron, in the proportion of a hundred pounds of tar to one hundred and eighty pounds of Portland cement. Care must be taken to heat the tar gently, and to mix the cement with it gradually, in order to prevent its boiling over. This mixture of tar and cement (wood cement) must then be laid as hot as possible on the paper with a tar brush. The next layer of paper is then laid upon it, and smoothed with a light wooden roller. In this way the whole roof must be covered. In order to break the joints of the paper, begin the second layer with half the breadth, and proceed as before. The third and fourth layers are,in like manner. laid with alternate layers of wood cement and brown paper. The last layer must be carefully covered with the cement, and then strewed with sifted ashes to the thickness of a quarter of an inch. Next to the gutter is a board, covered with zinc and projecting about two inches. It should be laid on after the second layer has been completed, so as to be covered by the third and fourth. If there are any chimneys projecting through the roof.they should be surrounded with zinc immediately after the first layer has been finished, and before the gravel is strewn upon it. This zinc should rise six inches mp the sides of the chimneys and three inches upon the roof; ine upper edges should be bent, so as to be let into the joints of the brickwork, where they should be carefully fixed with cement. By this means any water that may run down the outside of the chimneys is diverted to the roof. 3d. The whole is then finished with a coating of sifted gravel containing about one third of dry loam, truly leveled with rakes and scrapers. This work should not be attempted in rainy or frosty weather. The workmen should wear very light boots, or, better still, none at all, and should always stand on thin boards when working at the roof. The advantages of this system of roofing are : 1. A smaller quantity of wood is used. 2. The roof, being flat, gives more room in the upper floors of the house. 3. It is more convenient for constructing garrets. 4. Protection from external fire, and affords easy access to firemen. 5. If properly constructed, these roofs never require repair. Several roofs, at Hirschberg, in the Reisengebirge, constructed on this principle, are now twenty-two years old, and have never been repaired. Economy in Iron Manufacture. It is the determination, says the London Mining Journal, of the people who have the management of the iron mills in Russia, to do their work upon the most approved plans. For instance, possessed already of steam hammers, of considerable power, they are, nevertheless, having these' supplemented by others of a force equal to any to be found in the most modern department of any British iron works. The tools which they are now using have been sent out from this country,and those which they will soon receive will,also, go from the same firm. A member of it has only just returned from making the requisite arrangement in the Muscovite empire. Illustrative of the circumstance, that at the iron works in Russia, the managers are keeping themselves abreast of all the latest improvements in this country,is the fact that at the same time that they are increasing their individual hammer power, ironmasters in Great Britain,who are occupied in chief part in the manipulating of rails, are simulianeously extending their operations in a like direction. Much economy results from care in this respect. Rails of large proportions and of higher quality than have hitherto been common, are demanded by foreign customers. In, the producing of these, at a moderate outlay, much saving is effected by the rapidity with which forceful concussion can be brought to bear upon the metal in its early stages of manipulation. Ironmasters, who, in this country, have long held a distinguished position in the rail trade, are determined that they will not allow themselves to be distanced in the competitive race by modern firms, either here or abroad. They are, therefore, giving instructions for hammers of a caliber which would, only a few years ago, have been thought altogether out of proportion to the work required, but which are now acknowledged to be requisite to be laid down. And the firms who are doing this have, at the same time, intimated that they will not hesitate to make further advances as need may require. Cause for gratification is found in the circumstance, that in the iron works of this country, the steam hammer, in its varied shapes, is supplanting, in not a few instances, the old helve. There is one extensive iron works in this country/in which there is not now, I think, a helve to be found. The notion which iron-works managers of the old school clung to for a long time, is being exploded. It is now admitted by men who know most upon the subject, that better work can be done with the steam hammer than with the helve, even where much dross has yet to be beaten out of the iron. Tlien the immensely greater advantage which accrues from the use of the steam tool, when the blow has to be modulated, gives it a place which cannot be occupied by the helve. Most of the firms who produce these hammers are doing more in that branch of their trade than has marked their operations for some time past. The New Zlrconla Uglit. Three or four months ago, says the Mechanic's Magazine the news spread in England, through the medium of the scientific newspapers, that a discovery had been made in France, which would have the effect of abolishing the lime light by substituting zirconia for the lime cylinder. The advantages were stated to be that zirconia is not eaten away by tlie oxy-hydrogen flame, and that when not in use, it does not absorb moisture and crumble to pieces like lime; also, that in consequence of this stability, the ordinary clockwork of oxyhydro-gen lamps to turn the lime cylinder would be unnecessary with zirconia. It was further said, that the zirconia gave more light than lime under the same oxyhydrogen flame. Considerable interest in the new invention was, consequently, raised in this country, among the many who use the lime light, but weeks passed away without anybody being able to procure the zirconia cylinders in London. One night, however, at a soiree at King's College, the zirconia light was exhibited burning with great steadiness and brilliancy, in the presence of Professor W. Allen Miller, F,R.S., and many others, but no accurate tests were made, and both then and afterwards, the zirconia cylinders were as unprocurable in London as ever. Three weeks since, however, one of the first zirconia lamps procurable for examination in this country reached London, and was sent by Mr, R. J. Fowler, the Parisian correspondent of the " British Journal of Photography," to Mr. John Traill Taylor, the editor of that journal, with the request that he and Mr. W. H. Harrison would test its working qualities. The lamp was the property of Messrs. Harvey, Reynolds, and Co., Leeds. Accordingly, some experiments with the lamp were tried at the workshops of Messrs. Darker Brothers, philosophical instrument manufacturers, at Lambeth. At present, the French company refuses to sell the zirconia cylinders without their lamp be also purchased. According to the " Engineer," this lamp made for special use with the zirconia, gives a vertical flame, and the piece of zirconia is held in it by a little brass support. The piece of zirconia was I excessively small—about as big as a pea—and here at once was a source of great loss of light, because the flame was competent to raise to whiteness several times the area presented to its action. On this account alone, the total amount of light was very much less than the same flame gave with a lime cylinder, so as to put competition between the two out of the question, unless the zirconia surface be very greatly increased in size. The experimentalists then cut down a piece of lime till it equaled the zirconia in eize, and the lime and zirconia were exposed in turn to the flame, the result being that the zirconia was found to emit a less white and brilliant light than the lime under the same conditions, nor did variations in distance from the nozzle of the jet alter this result. Next, many variations in the pressure of the gases were tried, but the result was not altered. Then, substituting an Englisk " blow- through " jet for the blow-pipe sold by the French company, the same inferiority of the light from the zirconia was perceptible, nor did variations of pressure aflect the result. Lastly, a good orthodox oxyhydrogen blow-pipe was tried, wherein the two gases mix thoroughly some little distance behind the nozzle, and again the results were the same. These conclusions do not in any way affect the question of the permanency of zirconia under the fierce heat of the oxyhydrogen flame; but such permanency, if purchased at tie expense of inferior light, is teo dearly bought, and will condemn the invention. Unless the inventors are acquainted with some peculiarities of zirconia unknown to those who are versed in the use of the lime light, and can by an unknown method bring out a light from the zirconia equal to that given by lime, the zirconia light, from an economical point of view, is a failure. A few other experiments were tried, showing that soft lime and hard lime have to be placed at different distances from the blow-pipe nozzle to get the maximum amount of light from each. Chemical composition even more than hardness varies the amount of whiteness of the light. Magnesia cylinders were found to take a longer time to heat to whiteness and a longer time to cool than either lime or zirconia. Quartz rapidly vitrified under the flame, and asbestos could not resist the intense heat. It requires time and repeated heatings and coolings to test the permanency of zirconia under the oxyhydrogen flame to ascertain whether it does away with the necessity for clockwork apparatus. The piece used looked at the close of the experiments none the worse for the operations it had undergone, and a native zircon crystal, which, on previous occasions, Messrs. Darker had occasionally ignited under the oxyhydrogen blow-pipe, is now as hard as ever, having shown no tendency to crumble or srofteu like lime beneath atmospheric influences. The heat Lad produced in it traces of vitrification, wliich could bo saen only by the aid of a lens. Pliotograplis witli a White Surface, Put into a small mortar a teaspoonful of kr.olin, add thereto about a quarter of an ounce cf sensitive collodio-chloride, and stir well with the pestle until it becomes a smooth pasto. Add to this three fourths of an ounce more of the coUodicn, and again stir, and pour the whole into a bottle with one or two drops of castor oil. Shake well, and place it aside until the coarse particles have subsided. Edge a piece of talc or glass for about a quarter of an inch all round with dilute albumen, afterwards coat with the kao I'in collodion, and dry by gentle heat, when the talc or glass, if placed upon a piece of white paper, will have the appearance of alabaster. If the film splits, it should have a trifle more castor oil in the collodion; but the best remedy is to choose a more powdery collodion. If the film is upon glass, the progress of printing may bs examined from the back; but if talc be the medium used, it may be turned back in the same manner as when printing upon paper. Tone, fix, and wash in the same manner as with an ordinary collodio chloride print upon opal glass, and mount in a framo or case, to protect the picture from being scratched. It must not be varnished. After three years' trial the film has been found not to crack or leave the talc or glass after the picture has been ones finished. Many pretty effects may be produced by putting different colored papers behind vignettes produced in this way, as whatever color is placed behind the picture gives a delicate tinge of that color to the picture. Talc may be obtained in sheets as large as ten by eight inches.—Charles Durand in the Photographic Ntws. Collodion Varnish for Photo Prints, A very effective and agreeable polish is communicated to card or cabinet prints, etc., simply by coating them with a glutinous plain collodion. This polish is not so flagrant on the one hand as the so-called enamel surface, nor so dead as an ordinary albumen print that has undergone all the operations up to the mounting. I think I am justified in recommending the operation. Prepare the collodion as follows : PLAIN COLLODION.—Alcohol 3 ounces, ether 4 ounces, py-roxyline, 42 grains. Dissolve and filter in the usual manner. The prints are first cut out to the proper size and floated on the reverse side upon clean water until they lie perfectly flat; then take one print at a time and place it on a piece of glass of the same size as itself, moist side downwards; it easily adheres to the glass... Let the excess of water drain off and remove all moisture from the picture surface; now coat it with the collodion and let it drain in tiie usual way, then dry it before the fire or in any manner which is most convenient. The operation is quick; and, it seems to me, the gloss is just about ngh.t.—Profmor John Tmler, MD., in tlie PMladel phia Photographer.