112 HERAPATH'S HYPOTHESIS. 



varying from four to forty miles per second, according to 

 the direction of their motion with respect to the earth, along 

 with their various sizes, will suffice to show why some of 

 these bodies are destroyed the instant they arrive at our 

 atmosphere, and why others arrive at the earth's surface 

 with diminished velocity. 



" I cannot but be filled with admiration and gratitude 

 for the wonderful provision thus made by the Author of 

 Nature for the protection of His creatures. Were it not for 

 the atmosphere which covers us with a shield, impenetrable 

 in proportion to the violence which it is called upon to 

 resist, we should be continually exposed to a bombardment 

 of the most fatal and irresistible character. To say nothing 

 of the larger stones, no ordinary buildings could afford 

 shelter from very small particles striking at the velocity of 

 eighteen miles per second. Even dust flying at such a 

 velocity would kill any animal exposed to it." 



In the summer of 1848 Joule had completed a very bold 

 and successful attempt to submit the dynamical theory of 

 gases to the test of experiment, and communicated the 

 results to this Society. In this attempt he accepts the 

 theory of Herapath as simpler than his own. He gets 

 over the mathematical difficulties by an assumption which 

 shows his insight into the subject, as it is exactly that 

 to which the mathematical solution of the problem would 

 have led him ; and from this assumption he calculates 

 exactly what would be the velocity of the particles of 

 hydrogen, supposed all to move with the same velocity, in 

 order to give the, experimentally ascertained, pressure at a 

 certain density and temperature of the gas. The result he 

 obtains is, for a temperature of 32° Fahrenheit, a velocity of 

 6055- feet per second, while the most accurate determination 

 that has yet been made gives it as 6049'. 



