380 PHILOSOPHICAL TRANSACTIONS. [aNNO 1708. 



Experiments and Observations on the Motion of Sound, &c. By the Rev. Mr. 

 Derham, Rector of Upminsler, and F. R. S. N° 313, p. 2. FoL XKVL 

 Translated from the Latin. 



§ 1. Of the Differences, in the Velocity of Sound, among Authors. — The dis- 

 agreement among the best authors concerning the velocity of sound is exhi- 

 bited at one view in the following table, which shows the number of feet passed 

 over by it, as assigned by several of those philosophers. 



Sir Isaac Newton g68 feet Princip. Nat. Phil. 1. 2, prop. 50. 



The Hon. Mr. Roberts .1300 Philos. Trans. N° 209. 



The Hon. Mr. Boyle . . 1200 Essay on Languid Motion p. 21. 



Mr. Walker 1338 Philos. Trans. N° 247. 



Mons. Mersenne. 1474 .... Balistic. prop. 39. 



Flamsteed and Halley. . 1142 



Florentine Academy. . . . 1148 Exper. Acad, del Ciment. p. 141. 



The French Observatory 1 172 .... Du Hamel, Hist. Acad. Rtg. 

 There is no great difference among the last three, though there is among the 

 others: the reason of which is plainly this, viz. either from the insufficiency of 

 the instruments, or the different distances at which the observations were made, 

 or because of the wind. 



1. The instrument employed by some of those philosophers was not a regular 

 movement, as a watch or clock, but a simple suspended plummet vibrating 

 seconds. But it is plain that a common plummet is niiuch less convenient and 

 accurate than a proper movement; because it is necessary, first to have the eye 

 employed in observing the flash of the gun, and next the plummet or pendulum, 

 which takes up some time, and causes confusion ; all which, together with the 

 slowness of our senses, and that of our capacity or attention, may cause a con- 

 siderable error, especially if 2, There is but a small distance between the sono- 

 rous body and the observer. Now it is manifest, that most of those celebrated 

 authors made their experiments only at the distance of a few feet, and measured 

 by the return of the sound or echo ; for some of them scarcely extended their 

 measures above 6 or 700 feet, and others again not above a mile. But I have 

 always observed, that there was an uncertainty in experiments made at so small 

 distances, even though a person should use the very best instruments ; and the 

 least error in such small di^stances is to be reckoned considerable; for perhaps 

 the pendulum has run over half its arch, after the last pulsation, from the first 

 emission of the sound; but that pulse is also brought into the account, as if 

 the vibration had been full and complete, or possibly we anticipate a vibration, 

 and after the sound has reached us, perhaps we reckon more or less than we 



