NATUKAL PHILOSOPHY. . 193 



this natural pressure, will rush to fill a vacuum with a velocity of from 1200 to 

 1400 feet in a second. It has been, accordingly, conceived that when a solid 

 substance, such as a cannon-ball or rocket, is moving through the atmosphere 

 with a greater velocity than 1400 feet in a second, a vacuum will be produced 

 in its rear as it passes forward. Sir Howard Douglas and other eminent au- 

 thorities on gunnery, seem to have adopted this theory ; but we perceive that 

 it is questioned and denied by Colonel Parlby, of the Bengal Artillery, who 

 considers it a popular error to suppose that there is any vacuum left behind a 

 solid projectile. It is well known that whenever there is a vacuum produced 

 in the air by thunder, by the violent detonation of any powerful chemical com- 

 pound, or from any other cause, sound more or less loud is produced by the 

 particles of air collapsing upon each other. If there were, therefore, any vacu- 

 um created behind a projectile in its flight, it would necessarily be accompanied, 

 during the whole of its passage through the air, with a noise resembling thun- 

 der, so long as its velocity continued to exceed 1400 feet in a second. While a 

 cannon-ball or other projectile remains stationary, the atmosphere will press 

 equally upon every part of the surface ; but supposing it to move at the rate of 

 700 feet in a second, or with half the velocity with which ah 1 is supposed to 

 rush into a vacuum, then the air will be a resisting medium in front, and an 

 equivalent assisting medium in the rear. Colonel Parlby conceives that, when 

 the velocity reaches 1400 feet, as the air is highly elastic, it rushes round the 

 ball with a whirlpool rapidity ; and that, under this high pressure in front, the 

 velocity with which the particles of air move to the rear of the ball will be 

 regulated by that pressure, and not by the uniform velocity of 1400 feet in 

 a second. Coupling the absence of any succeeding loud noise with the above 

 theory, Colonel Parlby comes to the scientific conclusion that there can not be 

 any vacuum left behind a projectile at a velocity at which we can propel it. 



EFFECTS OF GUNPOTVDEK EXPLOSIONS. 



At the American Association, Prof. Olmstead read a paper on the Wil- 

 mington Gunpowder Explosion of May 31st, 1854. On that day, as three 

 wagons from Dupont's Mills were passing through "Wilmington, Del., each 

 with 150 barrels of Gunpowder, about 12,000 pounds, they exploded, demol- 

 ishing buildings and destroying life. Such wagons were accustomed to pass 

 that route daily for fifty years. The regulations prescribed had fallen into 

 disuse. They had left the mill at distances of half an hour, but had got within 

 twenty-five feet of each other. Wishing to trace out analogies between this 

 explosion and some phenomena of tornadoes, Professor Olmstead wrote to 

 Bishop Lee, whose house was destroyed, and received in answer from his son 

 some interesting facts. The cause of the explosion does not appear, but it is 

 known that two of the men were smoking by the side of their teams. Some of 

 the phenomena were surprising. A splinter from a Venetian blind was blown 

 through an inch board, making as smooth a hole as if pointed with steel. Metals 

 were often displaced. The shoes were torn off the horses' feet, castors from fur- 

 niture, and hinges from doors, and a wagon-tire was torn off and straightened, 

 and one piece left on a hill a quarter of a mile off. Windows were destroyed 



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