370 ^ LECTURE XXXI. 



well as the g]:eatest rarefaction and retrograde velocity, happening at the 

 instant when it passes through its natural place. 



We are ready to imagine that very hard bodies transmit motion instan- 

 taneously, because we have no easy means of measuring the interval of time 

 that elapses between the action of pushing the end of a rod, and the pro- 

 trusion of an obstacle at the other end, or between the instant of pulling a 

 bell rope, and that of the ringing of the bell. But it is demonstrable that 

 in order to transmit an impulse in a time infinitely small, the hardness of 

 , the substance must be infinitely great, and it must be absolutely incom- 



pressible and inextensible by any force, which is a property not discoverable 

 in any natural bodies: the hardest steel and the most brittle glass being very 

 susceptible both of extension and compression. 



The least elastic, substance that'has been examined, is perhaps carbonic acid gas, 

 ^ or fixed air, which is considerably denser than atmospheric air exposed to an 



equal degree of pressure. The height of the atmosphere, supposed to be ho- ' 

 ■?■ njogencous, is in ordinary circumstances, and, at the sea side, about 28 000 



feet, and in falling through half this height a heavy body would acquire a 

 velocity of 946 feet in a second. But from a comparison of the accurate ex- 

 periments of Derham, made in the day time, with those of the French Aca- 

 demicians, made chiefly at night, it appears that the true velocity of sound 

 is about 1 130 feet in a second, which agrees very nearly with some observa- 

 tions made with great care by Professor Pictet. This difference between 

 calculation and experiment has long occupied the attention of natural phi- 

 losophers, but the difficulty appears to have been in great measure removed 

 by the happy suggestion of Laplace, who has attributed the cff'ect to the 

 elevation of temperature, which is always found to accompany the action of 

 condensation, and to the depression produced by rarefaction. It is true that 

 a greater change of temperature would be required than Mr. Dalton's experi- 

 ments on the compression of air appear to indicate; but those experiments 

 do not perfectly agree among themselves; and the observation which has- 

 been made in France, that a heat, sufficient to set tow on fire, may be pror 

 duced by the operation of a condensing syringe, seems to show that Mr. 

 Dalton's results are somewhat below the truth. In this manner the theory 



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