Boiler Testing and Boiler Examining, MESSRS. EDITORS :—I have often thought, when reading in the columns of the SCIENTIFIC AMEKICAN and in other papers, accounts of explosions of boilers, that the treatment to which they are subjected in testing them for the pressure they are to sustain is one cause of the explosions. It seems improper to subject a boiler, either new or old, to a hydrostatic pressure of 125, 150, or 200 pounds to the square inch, when in use it is not intended to carry more than 50, 60, or 80 lbs. steam pressure. Does not this excessive strain tend to permanently weaken those parts which under a more sen sible treatment would with safety withstand the working pressure ? Is there any necessity of testing the boiler so far above its intended capacity ? If the object is to detect faults, the after treatment, when the fault is detected, is rarely calculated to remedy it. If a leak shows itself in a seam, the calking iron and hammer is used; if under the head of a rivet, a similar process—hammering down the rivet head around its edge. If a sheet bulges because of the imperfect welding of its laminse, a stay is put in or a patch put on. It does not appear to me that either of these methods amount to anything; I do not approve of makeshifts and temporizings, I believe in going to the root of the matter; remove the faulty rivet, and put in one that fills the hole; put in a new plate, or fit the one already there to its fellow; do not put patches on a favulty plate or sheet. These remarks apply with greater force to old or secondhand boilers than to new, for no boiler in use deteriorates equally in every part, and it is not always easy to determine the exact point of weakness by the hydraulic test, as the very weakest point may be protected and defended by a deposit of scale, sufficient to resist the pressure when cold, yet liable to break up and expose the imperfection when heated. The hydraulic test as used by the legalized inspectors of steam boilers is, in most instances, a farce, ridiculous but for its possible consequences. The inspectors are frequently pretenders or self-deluded imposters, wise in their own conceit; and their certificate of inspection and safety lulls the fears and encourages the confidence of the owner, who might otherwise be reasonably anxious and exercise proper care. It appears to mes as an old engineer, that personal examination of the boiler internally, if possible, but, at least, externally, and carefully, is far better for ascertaing the condition of a boiler than the hydraulic test. If this duty was religiously performed, at least once a month, by those who run stationary boilers, and the blowing off occassionally attended to, I feel certain that the number of explosions would be materially reduced. J. H. L. New York city. Congelation of Water. MESSRS. EDITORS :—Having carefully perused in the columns of the SCIENTIFIC AMERICAN the descriptions of the various machines for congealing water, recently patented in this country and in Europe, all of which are troublesome and expensive to manage, and impracticable for general use among a large portion of the agricultural communities throughout the United States, I would suggest the following method of obtaining ice in warm climates, superseding the use of machines or chemical mixtures, involving but a trifling expense after the first outlay, and possessing the additional advantage of the process being conducted by any person above seven or eight years of age. Travelers from the East have informed us of the method the Hindoos employ during the winter months, when the temperature has descended to 40, or less, above zero; which is, to select a piece of level ground on their respective farms, of about four acres in extent, and, having made shallow excavations throughout the field of two feet square by four inches deep, with interval-ling walls, four inches in thickness, and having placed therein pieces of straw matting, covering the bottom and sides about an inch in thickness, at night they place in the cavities tin pans filled with water of corresponding dimensions with the cavities, namely, nearly two feet square by three inches deep, the water in which will, in the morning, have become solid ice, if the temperature has fallen during the night as low as 40 Fah. above zero. The meteorological explanation is, that the straw matting, inclosing a wall of air an inch in thickness, prevents the heat underneath it, that is constantly radiating from the interior of the earth, from escaping into space otherwise than by the uncovered partitions or walls surrounding the cavities, which act as funnels conducting the radiating heat into space, before it haB time to sensibly affect the water in the tin pans, which latter assumes the solid form, from the lack of the supply of caloric, necessary to maintain its fluidity, that is constantly radiating throughout unimpeded space and matter. Experience has probably determined that the area of cavity, etc., here given, is the most suitable for the successful operation of the phenomenon. Any farmer throughout the warmer latitudes may, by thus understanding the nature and reason of the process (the sole varying condition being the attainment of a tern- j perature not above 40 Fah.), secure an abundant supply of this necessary luxury whenever desired, at a comparatively small expenditure of time, expense, or trouble. New YorS city. H. M. E. ! Increase of Resistance as Velocity Increases. MESSRS. EDITORS :—In the article signed " Mathematician," ! in your issue of January 30, 1869, at page 70, on the question, " Does Resistance to Ships Increase as the Square or Cube of the Velocity?" there is a confusion of ideas arising from omitting to include time as an element of the power required to overcome the resistance. It will require only four times as much steam to be developed and utilized in driving ' a steamer from New York to New Haven in one hour, as will be required if the voyage is performed in two hours; but the i engine and boiler which perform the voyage in one hour : must have the capacity to develop and utilize 8 times the steam per hour, since it is to overcome 4 times the resistance I in half the time. In other words, the resistance is as the square oi the velocity, but the power of the engine must be as the cube of th-3 velocity. j Following this article a quotation is made from " Silliman's : Principles of Physics," as follows : " The resistance increases as the square of the velocity; ior, if the velocity is doubled, the loss of motion must be quadrupled, because there is twice as much fluid to be moved in the same time, and it has also ] j to be moved twice as fast." There is in this paragraph a confusion of ideas somewhat similar to that before referred to. The single fact that " the fluid has to be moved twice as fast" is alone sufficient to quadruple the resistance. The further fact that " there is twice as much fluid to be moved in the ! same time" does not add to the resistance in the proper sense ! of that term, but it makes it necessary that the power of the engine, which has already been quadrupled for the former reason, should be doubled for this reason. Thus, as before, the resistance is as the square of the velocity, but the power of the engine that overcomes it is as the cube of the velocity. Required Power for Increased Speed of Steamers, j MESSRS. EDITORS :—Your correspondent of March 6th, page j 151, cannot discover my position on this subject, as published in an article of Feb. 20th, page 119, wherein I stated that the required power, or steam, was for a given distance, as the square of the velocity, and for a given time, as the cube. I gave a practical example of a steamer's ordinary time of passage from New York to Liverpool, being ten days; and to perform it in five days, by double velocity, would require a supply of four times the coal and steam over the former ten days' passage, or as the square of the velocity; but that during the five days' time in which the distance was made, the consumption of steam would be at the rate of eight times the former ten days' passage supply, or as the cube of the velocity. I am charged with a supposed error in omitting the important item of the time occupied, being only one-half " which he would not have done, had he been a more accurate mathema- I tician." If we try fig'ures, that " won't lie," and place the quantity of steam used on the ten days' passage at 1 per diem, we have 1 X 10 = 10; and during the five days under double velocity at 8 per diem, we have 8 X 5 = 40; or four times that of the ten days' passage, being as the square for the distance, and eight times, or as the cube of the velocity for the time, as previously asserted. T. W. BAKEWELL. j Pittsburgh, Pa. Noiseless Air Guns, j MESSES. EDITORS :—My attention was drawn, a short time j ago, to a paragraph in one of our dailies, on air guns, assuming their noiselessness and consequent adaptation to the assassin's purpose. The following is the description :—" It consists of lock, stock, barrel, and ramrod. The stock is made hollow, and provided with proper cocks for filling it with compressed air by means of a force pump. Each lock is nothing but a valve which lets, into the barrel a portion of the air compressed in the stock, when the trigger is pulled. The gun is loaded with wadding and ball, in the ordinary way, and the air, suddenly introduced from the stock, propels it with a velocity proportional to the square root of the degree of the of the compression of the air. By this weapon a person may be killed at a distance of sixty or eighty yards. Later im provements give it a propelling force almost equal to the old-fashioned musket. Its chief advantage to criminals is its noiseless discharge." It is surprising that such statements should find currency when they are so self-contradictory. In your valuable paper, page 57, No. 4, Vol. XVII., this subject is treated, and the notion of the noiselessness of air guns effectually disposed of. Now, that projectiles may b, thrown with deadly effect, almost noiselessly, is beyond dispute. It was one of the most ancient methods of warfare, and even now, and in this country, the fatal effects of the Indian's arrow receive almost daily illustration; and a bullet, or other form of projectile may be also impelled with great force by the bow or some other modification of the spring, The air gun is nothing new; every schoolboy has used the quill air gun, loaded with its potatoe disk; but it has its explosion—it is not noiseless. Now, in the air gun, the air is compressed, and it is a well-known fact, that compressed air or gas cannot be suddenly liberated against the atmosphere without producing a detonation. But while exploding gunpowder exerts a force against the air of about twenty thousand pounds per square inch, air cannot be compressed by mechanical means more than about forty times, or to exert a force of six hundred pounds per square inch; consequently, the effect of the projectile impelled by the compressed air and the detonation produced are less than those of gunpowder. New York city F. W. B. Patent Office Fees. MESSRS. EDITORS :—I hope you will excuse me for referring to the following interesting question, " Is Our Patent System Defective ? " found on page 105 SCIENTIFIC AMERICAN. I am impressed that this is a question of profound interest to all classes of mechanics. At the present time there is a great deal said and published in regard to extravagance, and advo- I eating reform and retrenchment in all branches of the GOT ernment, but nothing is said about reducing exorbitant fees, 1 high salaries, etc. I hope the SCIENTIFIC AMERICAN will become a strong ad -i vocate for reducing the patent fees, which I think are entirely : too high. This is an important question. Mount Olive, Va. L. PITMAN. [Certainly they are, and we should be glad to have the fees reduced; but the danger is, that when Congress commences to tinker the patent laws, we shall be saddled with a more complex and costly system. Reforms in legislation usually proceed very slow.—EDS. To Find the Contents of a Cylinder In Gallons, MESSRS. EDITORS :—As a good many of your readers may sometimes be called upon to find the capacity of a cylinder in gallons, permit me to offer you a rule, which I think is new am 1 short, and as near correct as in most cases may be wished for. It is: Multiply the diameter by diameter and hight of cylinder, and divide the product by the number 294, which I have found to be nearly correct—taking 231 cubic inches to thy gallon. Anything better will bo thankfully received by a good many of your readers. M. J. St. Richmond, Va. Crank Pin. MESSRS. EDITORS.—Friend Watson, asks, on page 151, of your issue of March 6, why an inside connected locomotive engine must have a crank-pin so much larger than an out-i side connected one. Suppose he puts his question in the following form, when it will almost answer itself; viz., Why does the axle of a locomotive need to be larger than the crank-pin ? CAI/LIFERS. Worcester, Mass. * ' Hallway Restaurants, It is an astonishing thing that, with scarcely an exception, there is not a railway restaurant properly conducted iu the United States. There are, indeed, no end of cake and pie shops; places where viscous and glairy pies, likewise doughy cakes, are to be had unlimitedly, but of honest bread and beef, clear unadulterated coffee, and tea that grew in China, there is very little; and the traveler with a simple stomach may j starve for aught the restaurant can do for him. Doubtless I there are people in the world who live and thrive on pie, dough boiled in fat, and similar edibles; but there are still others who, when hungry, satisfy their appetites with bread and beef, and some provision should be made for such ridiculous tastes. Along the line of the New York Central road are huge restaurants, one especially at Utica, where the eye rarges up and down immense tables covered with platoons of cakes dec-orated in the highest style of art; small cones of dough with holes in the top like volcanoes, others rolled up in scrolls and i still others spotted, ringed, and streaked with red sugar. Pies are strongly represented also, but for that juicy sirloin from which one can get a generous slice, for well made, well baked bread, for the round of corned beef, one looks in vain, and those who cannot feed upon such trifles may go hungry. It is perhaps useless to complain, but for all that we shall lift up our voice against such places in the hope that there may be a coming man who will keep a restaurant at railway stations with clean, well cooked, simple, food, at high prices, so that he can make some money out of it. In that event the institution will be universal, for hosts of imitators will arise and establish themselves in every corner of the land. Cannot the Falls of Niagara Ibe inade to Kim tlie Machinery of Buffalo. A correspondent from Ann Arbor, has been thinking, like many others, about the utilization of the enormous, and now wasted power of Niagara Falls, He sends us the following description of a plan for transmitting that power to the machinery of Buffalo, which, though it may be objected to by some, contains some good suggestions. He says : " First, I would make a proper channel for conducting the water from the river above the Falls to the bank or precipice below, where a sufficient number of turbine wheels may be put in to get the power wanted. With the power brought under control by these wheels, I would, by a series of force pumps, compress air into a proper receiver, from which a large main, pipe may be laid to Buffalo, having branches connecting with the engines scattered in the various parts of the city in the same manner as gas and water are now conducted to buildings. " I can see no reason why air could not be compressed so as to give a pressure of four or five hundred pounds to the square inch (perhaps more) in Buffalo. Hence, much smaller engines might be used, which in many cases would be no small consideration. It would, of course, be desirable to obtain as high a pressure as practical as a proportionally smaller pipe might be used, and it would be difficult to lay a very large pipe so great a distance. It seems to me that this is an enterprise Which might be managed by a stock company, and made to pay large dividends. The running expenses would be very slight. I am not prepared to say what capital would be required to put it in operation. It would not, I think, be very great as compared with the profits likely to accrue from it." IT is stated that the injurious action of mercury upon those employed in the looking-glass manufacture, can be prevented by using one-half per cent of sodium in the merenry, while at the same time the saving of the quicksilver will compen-I sate for the coat of the sodium.