36 THE POPULAR SCIENCE MONTHLY. 



SENSITIVE FLAMES AND SOUND-SHADOWS. 



By W. LE CONTE STEVENS, 



PROFESSOR OF PHYSICS IN THE PACKER COLLEGIATE INSTITUTE. 



THE conception that sound is due to wave-motion in an elastic 

 material medium was first distinctly expressed in the six- 

 teenth century by Lord Bacon. He distinguished between local 

 motion in a medium and the propagation of this motion through 

 it, referring to the transmission of sound through both air and 

 water by way of illustration. For measuring the velocity of 

 sound in air he proposed a plan which has been repeatedly applied 

 since his time, that of firing a cannon and noting the interval be- 

 tween the flash and the report as heard at a measured distance. 



It is impossible now to determine how far these observations 

 may have been original with Bacon, or to what extent they may 

 have expressed the current knowledge of his time. They were 

 clearly apprehended by Galileo, who discovered the law of simple 

 harmonic motion and made the first well-authenticated experi- 

 ments on the relation between vibration frequency and musical 

 pitch. But it is to Sir Isaac Newton that we must give the credit 

 of first applying the wave theory rigorously to the phenomena of 

 sound. Assuming this theory, he showed the possibility of calcu- 

 lating what ought to be the velocity of propagation through any 

 medium of known elasticity. He deduced a formula which has been 

 found applicable to most media. In the case of atmospheric air it 

 failed, but because it required a correction dependent on certain 

 laws of heat which had not then become known. The correction 

 was made by Laplace, and the formula, as thus completed, is now 

 found to be applicable to all known gases. This was only one of 

 the many important principles established on a mathematical basis 

 in the " Principia," and published in 1687. 



Even before this date, the conception that light, as well as 

 sound, might be due to wave-motion seems to have been grasped 

 by a few thinkers. In 1665 a book on " Light and Color " was pub- 

 lished at Bologna, two years after the death of its author, Fran- 

 cesca Maria Grimaldi, a Jesuit priest and astronomer. In this he 

 recounts some interesting experiments, which did not, it is true, 

 lead him to the wave theory of light, but served as the basis on 

 which this theory was subsequently established. Similar experi- 

 ments were made soon afterward by Robert Hooke, the ever- 

 jealous rival of Newton, and by Christian Huygens, their distin- 

 guished Dutch contemporary. Huygens demonstrated that, if an 

 impulse be given to any single particle in a uniformly elastic ma- 

 terial medium, it must be propagated thence as wave-motion 



