382 PHILOSOPHICAL TRANSACTIONS. [anNO 1708. 



the different strength of gunpowder vary the motion of sound? 14. Whether 

 sound pass over the same space in the same interval of time on the tops of high 

 mountains, and in the bottom of valleys, or in the highest and lowest parts of 

 the atmosphere? 15. Whether sound in acclivities and declivities have the 

 same degree of motion ; or whether it descend from the top to the bottom of 

 the hill with the same velocity, as it ascends from the bottom to the top of the 

 same ? l6. Whether sound move swift in the beginning, and slower in the end, 

 as is the case in a great many other violent motions? 17. Or whether it be not 

 rather equable ? viz. moving in half the time over half the space, in a fourth 

 part of the time a fourth part of the space, &c. 18. Whether sound have the 

 same degree of motion in all climates, both north and south, in England 

 France, Italy, Germany, &c.? IQ. Whether sound pass from one place to 

 another in a straight line, or in the shortest way, or whether it move along the 

 superficies of the intermediate earth ? 



To determine all these inquiries, I caused guns to be fired from towers, and 

 other eminences, at the distance of 1,2, 3, to 8 miles, but the guns that served 

 this purpose best, were those atBlackheath, called sakers, whose flashes I could 

 see from the turret of Upminster church, and hear the report almost in all 

 weathers, and even in the day-time I could with the telescope observe the 

 flash. 



The following experiment was made at that place, viz. two guns, called 

 sakers, were planted near each other, with the muzzle of one turned towards 

 me, and that of the other from me, and on February the 13th. 1705, they 

 were fired every half hour, from 6 o'clock in the evening till midnight, a gentle 

 wind blowing directly against the sound; the time between the flash of each 

 explosion (which I could observe with the naked eye) and the report, was always 

 about 120 or 122 half seconds; for the report was double, the first, which was 

 weaker, reached in about 120 half seconds, and the other, which was stronger, 

 in about 122 half seconds; and in the same manner there was a double report 

 of each explosion, during the whole time of the observation. — ^This reduplica- 

 tion of the report seemed to be an echo, reflected from the mill of Blackheath, 

 or the houses thereabouts; and of this I had no reason to doubt, only it hap- 

 pened to interfere with the opinion of a certain friend, who supposed that no 

 echo could be heard but what was produced by phonocamptic or reflecting ob- 

 jects, not far off from the observer, and not by those near the vocal or sono- 

 rous body, or other distant objects. 



^ 2. Of a distant Echo, or the Repercussion of Sound at a Distance. — And 

 first I suppose, that an echo at a great distance, is not contrary to its laws; in 

 the next place, that the reduplication of the sound came directJy from £lack> 



