Large and Small Cart-Wheels. Messrs. Editors :—Your correspondent, " F. W. B.," in No. 22, current volume, page 842, in his comments upon my communication in No. 20, of same volume, makes an amusing misapplication of a well-known law of friction, to prove that the friction between the axlo 'and the hubs of cart wheels, moving* tho same distance, in the game time, with a given load, will be the same, whether the wheels are large or small. The law which he invoices in support of this paradoxical proposition is laid down in the books in these words: " The friction is entirely independent of the velocity of continuous motion." All that this law establishes, in relation to the friction bc-i tween the axle and hub of a cart wheel, is this : In moving the same cart, with the same load, a given distance, you will have the same amount of friction to overcome, whether it moves at a greater or less velocity ; because there is the samo amount of robbing between the axle and its " circumscribing* ! box or bearing," in the one case as in the other ; and it makes no difference whether that amount of rubbing is performed in a long or a short time. It is precisely this law that proves the correctness of my proposition ; viz., that "by doubling the slzo of the wheels, you reduce the friction one-half/' ] To illustrate : Suppose the axle, on which the wheel turns, ! is six inches in circumference. It is manifest, that at each revolution, every particle of ma'.ter in tho hub or box, which j comes in contact with theaxle, must move around the latter j a distance of six inches, and with the friction due to the 374 weight of the load. Now, with wheels 6 feet in circumference in moving sixty feet there will be ten revolutions, and the surface of the hub or box in contact with the axle will travel around it the distance of five feet. But if you substitute wheels 12 feet in circumference, the wheels will make but 5 revolutions in moving 60 feet, and the rubbing surface of the hub or box will travel around the axle only a distance of two and a-half feet. The weight or pressure will be the same in both cases, and, consequently, the friction of each revolution will be the same, whether made in a longer or shorter time. The law may be expressed in these words: If you move one surface over aiid in contact with Another surface, under a given amount of pressure or weight, tha fric ion to be over-dome will be in proportion to the weight or pressure, and the distance wliich the moving body travels, without reference to the" time occupied in traveling that distance. I respectfully refer your correspondent to " Appleton's Dictionary of Mechanics," Vol. I, page 717, where he Will find. the law applicable to this subject clearly laid down, and fully sustaining my proposition. After falling into the error which I have above pointed out, your correspondent goes on to show, that there is an " advantage " in large cart wheels over small ones, independently of any saving of friction ; " and this advantage " he says, " de pends on the road, whether there are obstructions, like Stonw, sand, mud, or the settling down of the road bed under the wheels," etc. This is begging the question. In my communication I did not say that the saving of friction was the only advantage gained By using large wheels instead of small ones, in traveling over common roads. My assertion was, that the difference in friction between the axle and the hubs, " is the only reason why a horse can draw, on a level piano, a heavier load, at the same speed, on large wheels than on small ones." I adhere to that assertion, it being understood, of course, that I mean an absolutely level plane, when there are neither obstructions to surmount, nor depressions into which the wheels may sink. But a cart moving over a road obstructed by stones, mud-holes, ruts, etc., does not move on a level plane. It must inevitably have its " ups and downs." That large wheels will move over obstructions easier than small ones, is a propositien which I have never denied. Washington, D. C. J. J. C. Is Machinery Hostile to Mental Culture. MESSRS. EDITORS :Civilization always advanced in direct ratio to mechanical development ; the remains of ancient Egypt, Greece, Rome, China, Peru, Mexico, and everywhere else, prove it incontrovertibly. As the laws of nature revealed themselves to men, they grew intelligent, and while some used the knowledge obtained for improvements in industries, others made it their aim to further explore the recesses of nature, from which all wisdom flows. In either case as the necessity for improved mechanical means became urgent, ingenuity was taxed to supply the want. Thus we have the progress in civilization through industry by mechanical means, deducted from the laws that rule the universe. Machinery, therefore, is the promoter of human progress, the great lever by which we open the portals that exclude our vista from the formerly unknown, and therefore mysterious regions, enlarges our knowledge, and dispels ignorance and intolerance. Progress in knowledge is the certain road to perfection, to virtue, to further development of that intelligence in mankind, which only requires encouragement to expand over the immeasurable extent of the universe, finding there revealed the true source of all being ; it directs to morality, to rectitude, through justice. On the other hand, the substitution of automatical work for hand labor relieves the mass from a great deal of soul-benumbing drudgery, gives each more time to reflect ; and the observation of the numerous devices employed in itself promotes study, reflection, independent reasoning; the real and only source of true liberty, if joined to morality and j astice. R. H. New York city. Excellent Copying Ink. MESSRS. EDITORS :In your issue of May 15th I notice a lecipe for a new copying ink. Perhaps it may gratify some of your readers to be acquainted with another recipe which was published by me, in 1862, in Wteck's Illustrated German Polytechnic Gazette, and which will be found perfectly reliable. Take one half of a pound of extract of logwood (Sanf ord's is"] best), two ounces of alum, four drachms of blue and as much of green vitriol, and one ounce of sugar; boil these ingredients with four pints of water, filter the decoction through flannel, and add to it a solution of four drachms of yellow chromate of potassa in four ounces of water, and finally two ounces of chemic blue in two ounces of glycerin. The chemic blue, also called " blue dye," is the solution of indigo in oil of vitriol, and otherwise used for dyeing wool. You will notice that my composition differs from that given by you, in containing alum, instead of carbonate of soda, and sugar instead of gum arabic. Beside the ingredients of your ink, it contains chemic blue, and green and blue vitriol. In using these two salts I intend to effect a combination between ' them and the tannin of the extract of logwood. Your ink will probably just flow as well with one quarter less glycerin and one half less water of the quantity indicated. New York city. ADOLPH OTT. Why Large Wheels are of Lighter Draft than Small Ones. MESSES. EDITORS :Your correspondent" J. J. C," on page 811 of present volume, in answer to " F. R. P." criticising the latter's manner of explaining the reason why a cart with large wheels is of easier draft than one with small ones, gives an opinion I differ from as well as from that of " P. R. P." The cause is change in the angle formed on the one side, by the line of draft from the axis of the wheel, and on the other side from the axis of the wheel to the top of any object in front and against the wheel. The axis of the wheel TTeing the apex of the angle, it will be seen that the smaller the wheel the more acute this angle will be; the line of draft "being then lowered conieS udore behind the object to be overcome and increases the draft. If the wheel be so small that the line of draft coincides with the line of resistance the cart cannot be moved at all; " J. J, " says that a cart with wheels half the size of another will have double the friction at the axis' because it motes twice as far in going the same distance as the large wheels, but " J. J. C." must recollect that draft has twice the leverage on the small wheels that it has on the large ones, therefore in this respect they would be equally balanced. Gf. B. Princeton, Ind. Extinguishing Kerosene Lamps. MESSRS. EDITORS :Porthe last ten years, I have hardly ever read a single number of the " Scientific American," without feeling,that it was well worth the price you charge for a whole year's subscription. E.G., in the simple matter of extinguishing kerosene lamps; to have the safest, easiest, and best plan, is worth more to any family, using lamps, than the pittance paid for your paper. In No. 8 of the present Vol. of your excellent paper, we read—" To extinguish a kerosene lamp safely, turn the wick down until the flame is low and blow under the glass." In No. 10 of the same paper, we read— " Turn the wick up so as to produce a large flame, but not high enough to smoke ; then blow squarely across (not down) the top of the chimney." In No. 14 we read—" Turn the wick down until it is out, then turn it up ready for lighting." In No. 21 we read—" A kerosene lamp will be found extinguished in less than one minute from the time of complete disappearance of wick below the edge of tube through which it passes." I think the above plans obj eetionable.—First, because by " raising the wick before blowing out," the flame will immediately run down to the tube and thereby injure the quality of the wick for afterward conveying the fluid to the blaze. Second, because " lowering the wick to extinguish the lamp," will produce a kind of gammy substance in the upper part of the tube, which will ere long interfere with the raising of the wick when a new supply is needed. Third because " blowing under the the glass" takes such hard blowing and throws the blaze and smoke against the side of the chimney and soils it. Fourth because " blowing down the chimney " is unsafe and also tarnishes the glass. Other objections might be given, but let these suffice. After experimenting in the matter, I think I can give an easier, quicker, and safer plan than any of the above, for " extinguishing kerosene lamps." It is simply this:Blow across the top of the chimney, without either raising or lowering the wick. Let the blowing be a kind of puff and inclined upwards, so that no part of the blast will go down the chimney. This "plan needs no previous or subsequent fixing of the lamp. Try it. GEO. BUCHANAN Washington, Pa. Vibration of Metallic Vessels Containing Water. MESSRS. EDITORS :On a recent visit to Port Sullivan, Milam county, Texas, my attention was called to a curious fact bearing on this subject. The college bell had been taken down from the tottering belfry, and placed, with its frame, upon the floor of the portico, where it was still used for college and church calls. Some of the mischievous students turned it up, and propped it, and then filled it with water. Its diameter is about 18 inches, and its contents some five or six gallons. They then undertook to ring the bell by slight blows of the clapper against its walls. They, however, got little response; and after a few blows it was discovered that the bell was cracked in several directions. In fact, the pieces came asunder after emptying the bell, and showed the bell metal to have been of the most compact quality. The fracture was granular, but each grain clear and glistening. "What was the cause of the fracture ? The bell was accustomed to much more violent blows for years before." To the professor who asked this question, the writer gave this extemporaneous reply, without being very confident that it was satisfactory. " Instantaneous vibration against the water Inside was probably impossible, and hence the momentum of the blow forced a rupture ; or more specially, when the clapper struck the concave rim of the bell, there should have been in the open air, or any elastic medium, an instantaneous yielding of the concave in the direction of the blow, and a corresponding retraction on the opposite end of the diameter, and the circle for the moment would have assumed1 an ovate form. But as water is practically inelastic, the yield to the blow is not compensated by retraction and change of form; and hence the bell would crack, probably at some point of minimum strength." Experiments may readily settle the question, but we have a great scarcity of bells in Texas, and cannot afford to make these tests. C. Q-. FORBHET. Galveston, Texas. Cosmos states that a committee has been formed at Copenhagen with the intention of erecting a suitable monument in honor of the great Danish savant, Hans Christian Oersted. A statue, representing the distinguished natural philosopher, is ordered to be made by a Danish sculptor, named Ferichau, and is to be placed in a prominent situation in Copenhagen.