Laplace and the Speed of Sound

OR A CENTURY and a quarter after Isaac Newton initially posed the problem in the Principia, there was a very apparent discrepancy of almost 20 per cent between theoretical and experimental values of the speed of sound. To remedy such an intolerable situation, some, like Newton, optimistically framed additional hypotheses to make up the difference; others, like J. L. Lagrange, pessimistically confessed the inability of contemporary science to produce a reasonable explanation. A study of the development of various solutions to this problem provides some interesting insights into the history of science. This is especially true in the case of Pierre Simon, Marquis de Laplace, who got qualitatively to the nub of the matter immediately, but whose quantitative explanation performed some rather spectacular gyrations over the course of two decades and rested at times on both theoretical and experimental grounds which would later be called incorrect. Estimates of the speed of sound based on direct observation existed well before the Newtonian calculation. Francis Bacon suggested that one man stand in a tower and signal with a bell and a light. His companion, some distance away, would observe the time lapse between the two signals, and the speed of sound could be calculated.' We are probably safe in assuming that Bacon never carried out his own experiment. Marin Mersenne, and later Joshua Walker and Newton, obtained respectable results by determining how far they had to stand from a wall in order to obtain an echo in a second or half second of time. But Mersenne also used a gun, comparing the time of travel of the flash and the report; and all of the rest of the experiments listed below used this same technique (with the possible exception of Robert Boyle, who did not give reasons for his value). By 1660 the Florentine Academy had made careful measurements using a gun at a distance of about a mile. The results gave a value for the speedi of sound of 1077 Paris feet, or 1148 English feet per second. From this. point on the experimental values agreed rather closely, and no one seems to have questioned them seriously during the succeeding century and a half of debate. Nevertheless, we should note that most of these measurements were made without close attention to temperature, pressure, and moisture content of the atmosphere, or wind velocity, and it was really not until the