Kindly keep your queries on separate sheets of paper when corresponding about such matters as patents, subscriptions, books, etc. This will greatly facilitate answering yom questions, as in many cases they have to be referred to experts. The full name and address should be given on every sheet. No attention will be paid to unsigned queries. Full hints to correspondents are printed from time to time and will be mailed on request. (12511) W. T. H. writes: In your Answers to Correspondents, No. 12470, it is stated that steel is compressible at the rate of 68 millionths per atmosphere of pressure. I have the apparatus to make a simple test of the ma tter, a cylindrical tank, of sheet in 12 ' iuches in diameter, with cone-shaped ends, having a capacity of about three cubic feet. The tin does not exceed 1/64 inch in thickness. My proposltiou was to pump 15 pounds per square inch air pressure into the tank and observe the increase in the girth when the pressure was on. I figured that if steel (or iron) was compressible (or extensible) at the rate meutioned, the girth of the tauk should be 3/8 of an inch greater when the pressure was on. I made the experiment three times. The first time there was apparently an elongation of about 1/16 inch. The secona and third times I used more care, keeping the tape measure in the same place and trying to strain it to the same tightness iu all the measurements. In the two last trials there was no observable difference in the girth before and after the pressure was on. Of course extensibility is not compressibility, but I assume that they are about the same in amouut, only in opposite directions. The experimeut seems to indicate that there is no appreciable extension of iron with a tensile strain of uearly 7,000 pounds per square inch. A. A compression of 68 millionths In volume per atmosphere of pressure tells nothing of the deflection in oue direction only under a strain. The co-effideut of elasticity of steel is 30,000,000, which means that one pound per square inch tensile stress will elongate the steel 1/30,000,000 of its length. Using the formula pd = 2 TS for your experiment, 15 X 12' = 2 X 1/64 X S and S = 6,000 pounds per square inch on the cross-section of the trimmed irou, 1164 in ch thick. This would elongate the circumference of the tank 6,000/30,000,000, or 1/5,000 of its 12' X 3.14 length arouud, equal to -----------------or 0.00785 5,000 inch-too small to measure. Evidently the seam of the tank yielded in the first experiment, and after the lost motion was taken up you obtained no appreciable effect from the 15 pounds pressure again applied. (12512) S. E. K. says: 1. Is there such a thing as “heat” lightning, or is all lightning “chain” lightning, and what we call heat lightning simply the reflectiou of a distant thunder storm ou the clouds? A. The lightning called “heat lightning” is usually due to the reflection of a lightning flash below the horizon upon clouds above the horizou. It is too distant for oue to hear the thunder from the flash. All lightning is not “chain” lightniug. The real existence of “ball” lightning is now quite generally admitted by men of science. 2. When we see oblique rays apparently coming from the suu, is this really the action of the suu “drawing water,” as the saying is 7 A. The appearance which is called “the sun drawing water” is caused by the sun's light passing through openings in the clouds and illuminating the particles in the air below, thus produciug light stripes iu tho space below. The same may be seen sometimes after the sun has set, the stripes there being directed upward. (12513) A. W. K. asks: Will you kindly iuform me through your Question Column of a good formula for wet cells? A. There are four prinCipal forms of wet cells; the one to be used depends upon the character of the service required. For interrupted, occasional work, such as bells, burglar alarms, and ignitiug gas and gasoline engines, the Leclanche cell is the best. Its solution is made by dissolving a quarter of a pound of ammonium chloride in water, enough for' one cell. The Edison cell is also used for ignition and similar purposes. Its solution is a rather strong solution of caustic potash. The proper quantity is furnished with the cell. For a steady constant current, such as is required for electroplating, charging small storage cells, or for a telegraph battery, the gravity cell is the best. Its solution is a saturated solution of copper sulphate ou the cOPIXlr plates in the bottom of the jar with crystals of the same salt in excess to maiutaiu saturation. And last, for giving a large curreut for a short time, such as for heating a wire, or for running a motor or workiug an induction coil for X-rays or a small wireless telegraph outfit, the best cell is the bichromate cell. The solutiou is a strong solutiou of bichromate of potash in sulphuric acid and water. The 'mode of preparing the solution as well as of making the battery is well described in SUPPLEMENT No. 792. A full description of all these cells is contained in SUPPLEMENT Nos. 157, 158, 159, whicb you should have if you ueed to use wet cells for any purpose. We will send the four papers for forty cents. Wet cells are not much used. The more convenient dry cell has taken their place. The dry cell is simply a Lelanche cell in which the solution is absorbed in some material so that it will not run out. The cell is then sealed to P'event evaporation, (12514) E. F. B. says: Can you tell me approximately how many vibrations per second are required to produce a sound so high iu pitch that it would be imperceptible to the average human ear? A. The average range of hearing is from 16 to about 40,000 vibrations per second. Some persous are not able to connect 16 vibrations per second into a continuous toue, and for some the inability to hear is below 40,000. This matter is tested by a Galtou's whistle, which can be tuned up to more thau 40,000 vibrations. (12515) J. D. asks: A filled a drinking glass full of water, placed a piece of paper over the top, held it firmly, reversed the glass paper side down, then removed his hand from the paper; the paper remained in its place, and the water did uot ru) out of the glass. A says it is due to the atmospheric pressure on the outside. B says it is vacuum aud atmospheric pressure. Which is correct? A. ''he experiment you describe Is explained by the surface tension of the water, which prevents the air from entering betweeu the paper and the edge of the gluss, and the atmospheric pressure, which holds the paper up and prevents it from dropping off the glass. It is not necessary to have the glass full of water. H only partly full, with air above the water, the experiment succeeds just as well. No vacuum is formed. (12516) F. D. D. asks: The Nautical Almanac states tha t June 21 st (astro. day) is the longest day of the year, being 15 hours 23 minutes (90 W. longitude, 43 N. latitude), but the earth does not reach aphelion until July 2nd, on which date the day is 5 minutes shorter than June 21st. I have always supposed that the axis of the earth tipped t.1ward the sun to its greatest degree on June 21st, and cousequently the earth must be at aphelion on that date. A. The sun is longest above the horizon on June 21st or 22nd” and hence this is the date of the longest daytime, or, as we term it in common speech, “the longest day.” This is because the suu is then farthest north. It is vertically over the Tropic of Cancer, and makes the longest daytime iu the northern hemisphere and the shortest daytime in the southern hemisphere. There are more degrees of the diurual arc above our horizou in June than in January or any other mouth of the year in the northern hemisphere. This does not depeud upon perihelion at all. The earth is In perihelion on January 1st to all parts of the earth alike, and in apheliou early in July. At perihelion the earth is traveling swiftest in its orbit, aud at aphelion it is moving slowest. At perihelion the sun therefore makes more right asceusion between noon and noon than at any other time of the year. For this reason the time from noon to noon is longest, mOre seconds, on Jauuary 1st. 'L'his you can easily verify by. the Nautical Almanac. January 1st is the longest day, from noon to noon, in the year. From this you will see that the longest day is a very different idea from the longest daytime. The longest day nearly coincides with the shortest daytime iu the northern hemisphere, but in the southern hemisphere the longest day and longest daytime are nearly coincident. The two are determiued by different conditions, and are not dependeut upon each other at all. The axis of the earth is for limited periods in positions parallel to itself. At the equinoxes, March aud September, the suu is vertical over the equator, and the axis of the earth makes a right angle with the line joining the centers of the earth aud the sun. At the summer solstice of the northern hemisphere in June, the earth's axis has come around apparently, so that its northern end is directed 23 degrees 30 miuutes over toward the sun. The converse of this is true of the southern hemisphere at the same time. At the winter solstice of the northern hemisphere the axis of the earth has shifted 47 degrees from its position as referred to the sun from what it occupied at the summer solstice. A small globe, even au orange, and a lamp will enable you to demoustrate all this experimentally. It is uicely described in Todd's “New Astronomy,” pages 150 to 160, a book which we send for $1.50 postpaid. Our circulation tal,es the SCIEN1'IFIC AMERICAN all over the world, and a description of these pheuomeua written for the uorthern hemisphere would be the reverse of true for the southern hemisphere. (12517) R. J. G. asks: Does an exhaust fan create a vacnum when runuing at full speed? A. An exhaust fan erea tes a partial vieuum ouly. It cannot draw all the air out of a room or spnce. The very best mechanical air pump cannot do that. 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Nowadays the chemist-magician or the engineer-sorcerer evolves a palatable wine jelly out of old shoes, alcohol out of discarded shirts. Love letters are written on paper made from an old linen cuff, with ink that owes its origin to the rusty hoop of a beer barrel. The swish of a silken skirt is but the transformed rustle of a tree-top in the wind, for much silk is made nowadays from cellulose. Undoubtedly the subject which is of most vital and personal interest to the American people is that branch of chemistry that deals with foods, drinks and drugs. At the time when the findings of Dr. Wiley had set the country astir, and the enforcement of the Pure Food and Drugs Act brought a storm of opposition from manufacturers involved, certain questions arose on which eminent authorities failed to agree. Accordingly President Roosevelt appointed a Board of scientific experts of the highest standing in the country as a supreme court whose decisions should be fnal and beyond further question. Ira Remsen, 'President of Johns Hopkins University, will tell us how this Referee Board, of which he is the Chairman, came to be appointed, explaining certain facts not generally known to the public. America is such a young housewife that she has not yet learned the thrifty ways of her European neighbors. She is just beginning to learn that much of the stuff that formerly found its way into her garbage can is worth real money. What has, and may be done in utilizing our waste materials, will be told by Robert Kennedy Duncan, Professor of Industrial Chemistry at the Universities of Kansas and Pittsburg. Not only have we been prone to throw away valuable matter, but we have shown a surprising laxity as buyers. We are now waking up to the fact that it is most important to make thorough chemical tests of raw materials. On this subject Dr. Charles F. McKenna, a leading expert, has prepared an interesting article. Although we do not hold a very enviable position among the nations in the broad felds of chemical achievement, we may point with pride to our work in electrochemistry. Chemistry is magic, electricity miraculous. What these two mysterious forces may accomplish when working together makes a most engrossing chapter. William H. Walker, Professor of Chemistry at the Massachusetts Institute of Technology, has undertaken to write this narrative. A very special work has been taken up by Dr. A llen Rogers, of Prall Institute. He is going to explain how he trains men to become foremen in works employing chemical processes, thus making for intelligent co-operation in place of the ignorance and opposition with which the expert chemist has had so frequently to contend. Our readers are probably familiar with the platinum sponge igniter for gas jets. The mere introduction of the little device into the stream of gas issuing from a burner causes the gas to burst into fame. We have here one of the infinitely numerous examples of so-called “contact action” or catalysis, a phenomenon which plays a most important role both in industrial work and also in vital processes. Of its significance, and the mystery still surrounding it, which modern science has as yet been unable to dispel, Mr. A. ]. L,otka, writes in an article prepared specially for this mid-monthly issue. In addition to the above articles, the September 16th number will contain the usual Editorial, Aviation, and other Departments. RIBBON DENTAL CREAM I Cleans-preserves-polishes deli. ciously and antiseptically. Different from other dentifrices in its delightful flavor and double efficiency. It not only kills decay-germs when used but also leaves the mouth in that sweet, clean, non-acid condition which counteracts germ-growth. Delicious—Efficient Ribbon Dental Cream proves that a “druggy” taste is not necessary to efficiency. It is (he antiseptic, anti-acid cream-efficient without “grit." 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This article was originally published with the title "Notes and Queries"