386 PHILOSOPHICAL TRANSACTIONS. [aNNO 1708' 



and those measured by the motion of sound; so that the difference was either 

 nothing, or but a few centesimal parts; unless when the wind was favourable, 

 excepting at South Weal church, of which hereafter; so likewise in the ob- 

 servations made at the churches of Dagenham, Warley, Thorndon, and Bark- 

 ing, the distances, measured by the sound, seemed to be a little shorter, be- 

 cause the wind accelerated the sound; but in computing this column of the 

 distances by the sound, I made no allowance for the acceleration of the winds ; 

 only divided the number of vibrations, or half seconds, by Q-J-, or 9-25, the 

 number of half seconds that sound moves in a mile. The equal motion of 

 sound is also evident from this table, by comparing the vibration and distances; 

 or from the single column of distances by the sound. 



To confirm all this, I went to Foulney-sands, on the Essex coast, which 

 form a large and regular plain, of several miles in length : there I measured 6 

 miles, and almost at the end of each mile made experiments, by firing guns ; 

 by which I found that all my former observations were very just and true ; viz. 

 that sound moves a mile in 9 half seconds and -^y two miles in 18 and 4-j and 

 three miles in 27 half seconds and 4, and so on. 



^ 8. Of the Motion of Sound in Ascents and Descents ^ ^c. — As to the 1 5th and 

 19th queries, I confess I could not be satisfied by all the experiments made to 

 that purpose. And first, as to the progressive motion of sound in the shortest 

 line, quer. 19, I had reason to doubt of it, from the difference of the distance 

 as measured trigonometrically and by the sound, between Weal and Upminster, 

 as in the above table. The trigonometrical mensuration was taken so many 

 different ways, and with so large angles, as to leave no doubt on that head : 

 but because the distance seemed, by the mot on of the sound, to be greater, 

 and because the superficies of the intermediate ground is very uneven, I sus- 

 pected whether the sound had not a wavy or undulated motion ; or whether the 

 intermediate hillock did not repress and retard its undulations. In order to 

 solve this difficulty, in some measure, I caused a musket to be fired from the 

 top of Langdon-hill, into the valley below it, at 3.79 miles oflf: the distance 

 was accurately measured trigonometrically, by taking pretty large angles and a 

 base; and when the experiment was made, a gentle wind blew somewhat against 

 the sound: between the flash and the report I reckoned 35 half seconds and -|-; 

 which number so well accords with the distance, and so nearly agrees with the 

 other experiments, that there is no manner of doubt, but the sound descended 

 through the air, in a right line from the top of the hill to the bottom of the 

 valley, and not according to the incurvated surface of the intermediate ground : 

 and therefore I suppose that there was some small oversight in the observations 

 made at Weal ; because I neither observed any such thing in the experiment 



