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Steam Boiler Explosions.
MESSRS. EDITORS:--There have been various theories and opinions advanced in explanation of steam boiler explosions. Some of these may be sufficient to account for some explosions, but none of them account for all the explosions that have taken place. The only one that I ever read accounting for all was written by a lady (Miss Fanny Purves, of Philadephia), and that stated that it was nothing more than over pressure; and she counseled, as a preventative, the use of pure tough iron in plates sufficiently thick to bear this over pressure. A question here arises--What causes this over pressure? Is it the indefinite expansion of steam, under a continuous increase of temperature? Such is held to be the case by some, but that it not sufficient, because there is a limit to that theory. Perkins has shown by experiment, that if you take an iron vessel and fill it with water, and heat it to a red heat, and then open a hole, no steam will escape while the vessel and water are at this high heat; but that as soon as the temperature goes down, the steam will escape while the vessel and water are at this high heat; but that as soon as the temperature goes down, the steam will escape with great force. We know that when water runs out of the tea-kettle on a hot stove, it will dance about in globules, generating no steam. This is termed the spheroidal state of water, but the intrinsic philosophy of this spheroidal condition is not fully understood. There appears to be a balance between expansion and contraction, or, attraction and repulsion, governed by some law not philosophically understood; and it appears to me that it is this condition of the water in the boiler that causes those terrific explosions that frequently occur. It is well known that when steam in the boiler acquires a high pressure, its augmentation of heat and pressure follows rapidly; and it will not take many minutes to put the water up to the spheroidal heat. The boiler will have to bear an undue strain before the water attains its spheroidal heat; and if it leaves it, will no doubt be impaired more or less, but will nevertheless hold in the spheroidal water; because in this condition it makes no steam, and consequently exerts no pressure. Now while in this condition, the great danger is pending, and of this the engineer is admonished by the following symptoms: The steam whistle when worked will squeak, and suck, and fret; the safety valve will chatter and work asthmatically, if you will allow me the term, and the only remedy that I I can see is for all surrounding persons to flee the wrath to come. For as soon as the temperature goes down to the point at which the water gives up its spheroidal condition, and assumes that of high steam, there will be a sudden augmentation of steam and pressure greater than it was when it assumed the spheroidal form, and then follows that terrific explosion we too frequently hear of. I could cite a number of cases of boiler explosions corroborating this view of the matter. The opening of the safety valves by outside force lets in cold air by suction; for in the spheroidal condition of the water the boiler will suck, or inhale, instead of pressing outward or exhaling. And so with the steam whistle, it will wheese, and suck, and fret, and chatter, giving warning that an unusual condition of things exists inside the boiler.
When we shall learn the electrical condition of the after in its spheroidal condition we may devise some means to avert the calamitous consequences of its changes in steam boilers; for in all the explosions that I have investigated, I found that anomalous condition of the safety valves, steam whistles, and gage cocks. 
Professor Hare, of the University College at Philadelphia, made some experiments many years ago upon the quiescent condition of water in a red hot vessel; but could not arrive at any well-established conclusions. And now, since it is only a step from high steam to spheroidal water, and a back step from spheroidal water to higher steam, it seems to be a thing of too easy occurrence for destructive explosions, especially where smaller boilers are used for great powers-especially the tubular boilers. 
JOHN WISE.
Lancaster, Pa. 

APRIL 25, 1868
Scientific American.
THE WHEEL QUESTION.

"How many revolutions on its own axis will a movable wheel make in rolling once around a fixed wheel of the same diameter? Answer, [[italics]] One." [[/italics]]
This discussion, instead of subsiding, has, like Vesuvius, now broken out afresh, and is having an excited run in various parts of the country.
We were lately advised that the learned Professor of Engineering in one of our largest northern colleges, had pronounced in favor of two revolutions. The same mail brought us an excellent essay from the learned Professor of Engineering in a prominent southern establishment, clearly showing that there could be but one revolution.