VOL. XX.] PHILOSOPHICAL TRANSACTIONS. 339 



Mersenne mentions an experiment, in which he found the motion of sound 

 to be 1 174 feet in a second. And the Academy del Cimento caused six harque- 

 busses and six chambers to be fired, one after another, at the distance of 5739 

 English feet, and from the flash to the arrival of each report was 5" ; and re- 

 peating the experiment at the mid-way, the motion was exactly in half the time, 

 which gives 1148 feet per second. Mr, Boyle also mentions that he has more 

 than once diligently observed, that the motion of sound passes above 400 yards, 

 or 1200 feet in l". 



Mersenne and the Academy del Cimento conclude, that sounds are all of the 

 same quickness, whether they be great or small, and whatever temper the air 

 is of, though Mersenne was once of another mind : but Kircher, from several 

 experiments, infers, that loud sounds move quicker than small ones. Dr. Plot 

 also tells us, the echo returned the sound of a pistol much quicker than a voice ; 

 and that it repeated more syllables in the night than in the day : whence it follows 

 that the sound moved slower in the night than in the day. Kircher says, that an 

 echo, which in the night repeated 14 syllables, repeated only 7 in the day. Because 

 there seems to be so great affinity between the undulations of water, and the 

 propagation of sound, therefore the Academy del Cimento tried some experi- 

 ments about the first ; and they tell us, that the larger the stone is, which is 

 thrown into the water, and the greater the force, by so much is the undulation 

 swifter: though Gassendus had before affirmed, that the undulations of water 

 are all equally swift. And I have often observed, that when a stone has been 

 thrown into the water, the further the undulations removed from the centre, 

 the greater was the distance from one another, even of those that rolled the 

 same way : so that the motion of each precedent undulation was quicker than 

 that which followed it. If this may be allowed for any argument, it makes for 

 Kircher's opinion. By some of those experiments which I tried, I am inclined 

 to think that the sound moved quicker when it was calm, than in a wind, 

 even when the sound moved half way with the wind. Some other experiments 

 seemed to confirm an opinion of Kircher's, who says, that a sound moves 

 swifter at first than afterward, as is usual in other violent motions. 



In places where there are parallel walls, not above 6 or 8 yards asunder, 

 as in Trinity Ball-court, and at the entrance into St. John's-Grove, &c. I have 

 heard the echoes of a clap following one another distinctly enough ; but there 

 the echoes of a musical note, which was longer than a clap, were so confused, 



ments have but little accuracy in them. And we shall find hereafter that the more accurate experi- 

 ments of Dr. Derham, &c. give 1 142 feet for the velocity of sound per minute, which has ever since 

 been used as a standard number. The number 1148, found by the experiment of the Academy del 

 Cimento, agrees very nearly with this. 



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