ACOUSTICS. 



at the bottom of deep rivers, from hence 

 it would seem not to be very material in 

 the propagation of sounds, whether the 

 fluid which conveys them be elastic or 

 otherwise. Water, which, of all substan- 

 ces that we know, has the least elasticity, 

 yet serves to carry them forward : and if 

 we make allowance for the difference of 

 its density, perhaps the sounds move in it 

 with a proportionable rapidity to what 

 they are found to do in the elastic fluid of 

 air. But though air and water are both ve- 

 hicles of sound, yet neither of them, ac- 

 cording to some philosophers, seems to be 

 so by itself, but only as it contains an ex- 

 ceedingly subtle fluid,capable of penetrat- 

 ing the most solid bodies. One thing, how- 

 ever, is certain, that whatever sound we 

 hear is produced by a stroke, which the 

 sounding body makes against the fluid, 

 whether air or water. The fluid, being 

 struck upon, carries the impression for- 

 ward to the ear, and there produces its 

 sensation. Philosophers are so far agreed, 

 that they all allow that sound is nothing 

 more than the impression made by an 

 elastic body upon the air or water, and 

 this impression carried along by either 

 fluid to the organ of hearing. But the 

 manner in which this conveyance is made 

 5s still disputed: whether the sound is 

 diffused into the air, in circle beyond cir- 

 cle, like the waves of water when we dis- 

 turb the smoothness of its surface by 

 dropping in a stone ; or whether it travels 

 tdong, like rays diffused from a centre, 

 somewhat in the swift manner that elec- 

 tricity runs along a rod of iron ; these are 

 the questions which have divided the 

 learned. Newton was of the first opinion, 

 lie has explained the progression of sound 

 by an undulatory, or rather a vermicular, 

 motion in the parts of the air. If we have 

 an exact idea of the crawling of some in- 

 sects, we shall have a tolerable notion of 

 the progression of sound upon this hypo- 

 thesis. The insect, for instance, in its 

 motion, first carries its contractions from 

 the hinder part, in order to throw its fore 

 part to the proper distance, then it carries 

 its contractions from the fore part to the 

 hinder, to bring that forward. Something 

 similar to this is the motion of the air when 

 struck upon by a sounding body. All who 

 have remarked the tone of a bell, while 

 its sounds are decaying away, must have 

 an idea of the pulses of sound, which, ac- 

 cording to Newton, are formed by the air's 

 alternate progression and recession. And 

 it must be observed, that as each of these 

 pulses is formed by a single vibration of 

 the string, they must be equal to each 



other; for the vibrations of the string are 

 known to be so. Again, as to the veloci- 

 ty with which sounds travel, this Newton 

 determines, by the most difficult calcula- 

 tion that can be imagined, to be in pro- 

 portion to the thickness of the parts of 

 the air, and the distance of these parts 

 from each other. From hence he goes on 

 to prove, that eachlittle part moves back- 

 ward and forward like a pendulum ; and 

 from thence he proceeds to demonstrate, 

 that if the atmosphere were of the same 

 density every where as at the surface of 

 the earth, in such a case, a pendulum, that 

 reached from its highest surface down to 

 the surface of the earth, would, by its vi- 

 brations, discover to us the proportion of 

 the velocity with which sounds travel. 

 The velocity with which each pulse would 

 move, he shows, would be as much great- 

 er than the velocity of such a pendulum 

 swinging with one complete vibration, as 

 the circumference of a circle is greater 

 than the diameter. From hence he calcu- 

 lates that the motion of sound will be 979 

 feet in one second. But this not being 

 consonant to experience, he takes in ano- 

 ther consideration, which destroys entire- 

 ly the rigour of his former demonstration, 

 namely, vapours in the airland then hnds 

 the motion of sound to be 1142 feet in 

 one second, or near 13 miles in a minute, 

 a proportion which experience had esta- 

 blished nearly before. Many other theo- 

 ries on this subject have been advanced 

 by ingenious men, but our limits do not 

 allow to enter farther into them. 



Since by experiment it has been proved 

 that sound travels at about the rate f?1142 

 feet in a second, and that no obstacleshin- 

 der its progress, a contrary wind only a 

 small matter diminishing its velocity, the 

 method of calculating its progress is easily 

 made known. When a gun is discharged 

 at a distance, we see the fire long before 

 we hear the sound. If then we know the 

 distance of the place, and know the time 

 of the interval between our first seeing the 

 fire and hearing the report, this will shew 

 us exactly the time that the sound has 

 been travelling to us. For instance, if 

 the gun is discharged a mile off, the mo- 

 ment the flash is seen you take a watch. 

 and count the seconds till you hear the 

 sound, the number of seconds is the time 

 the sound has been travelling a mile. 

 We are also enabled to find the distance 

 between objects that would be otherwise 

 immeasurable. For example ; suppose 

 you see the flash of a gun in the night at 

 sea, and tell seven seconds before you hear 

 the report, it follows tkerefore that the 



