ACOUSTICS 



perhaps supply the place of foreign spices ; 



.UK! indeed it is the only native aromatic 

 plant of northern climates It is carmina- 

 tive and stomachic, and often used as an 

 ingredient in bitter infusions. 



ACOTYLEDONE8, in botany, plants st> 

 railed. because their seedsare notfurnish- 

 fd wii ; . I of course put forth no 



iemiii..l leaves. All mos-.es are of this 

 kind. See ( ' n-i , 



\( <U .sTM.'s,, in physics, isthatxcience 

 which instructs us in the nature of sound. 

 It is divided In some writers into diacous- 

 tic>, which explains the properties of those 

 sounds that conic distinctly from the so- 

 norous body to the ear; and catacoustics, 

 which treats of reflected sounds; but this 

 distinction is not necessary. Intheinfan- 

 c\ of philosophy, sound \v as held to be a 

 separate existence : it \vasconceived to 

 he wafted through the air to our origins 

 of hearing, vvhichit \vassupposcd to affect 

 in a manner resembling tliut in which our 

 nostrils arc- uifec.ted when they give us the 

 sensation of smell. Vet, even in those 

 early years of science, there \verc some, 

 and, in particular, the c.-lehrat'.-d founder 

 of the Stoic school, who held that sound, 

 that is, the cans.- of sound, was only tht 

 particular motion of external gross matter, 

 propagated to the ear, and there produc- 

 ingthat agitation of the organ, by which 

 :1 is immediately affected with the 

 '. n\ of sound, /eno says, " Hearing 

 is jrroduccd by the air which intervenes 

 between the thing sounding and the ear. 

 The air is agitated in aspherical form, and 

 moves oll'in \\a\es, and falls on the ear, 

 in the same manner as water undulates in 

 rircles when a stone has been thrown into 

 it." The ancients were not remarkable 

 for precision, either of conception or ar- 

 gument, in their dis< unions and they 

 \\crc contented with a general and vague 

 view of things. Some followed the opinion 

 nf /eno, without any farther attempts to 

 give a distinct conception of the explana- 

 tion, or to c-ompare it with experiment. 

 But, in latter times, during the ardent re- 

 searches into the phenomena of nature, 

 this became an interesting subject of in- 

 quiry. The invention of the air-pump 

 the first opportunity of deciding, by 

 experiment, whether the elastic undula- 

 tions of air were the CUMMS of sound ; and 

 the trial fully established the point ; for a 

 bell rung in vacuogavc no sound, and one 

 rung in condensed a';- \ loud 



one. It WAS therefore received as a. doc- 

 trine in general physics, that air \\ as the 

 <und. The celebrated (ialileo, 

 the parent of mathematical philosonhv. 



discovered the nature of that connection 

 between the lengths of musical chords and 

 the notes w hich they produced, which had 

 been observed by Pythagoras, orli 

 by him in his travels in the Kast, and which 

 he made the foundation of a refined and 

 beautiful science, the theory of music. 

 Galileo shewed, that the real connection 

 subsist ed between the tonesand tile vibra- 

 tions of these chords, and that their dif- 

 ferent degrees of acute u- .jondetl 

 to the different frequency of their vibra- 

 tions. The very elementary ami familiar 

 demonstration which he gave of this con- 

 nection did not satisfy the curious mathe- 

 maticians of that inquisitive age, and the 

 mechanical theory of musical chords was, 

 prosecuted to a great degree of refine- 

 ment. In the course of this investigation, 

 it appeared that the chord vibrated in ;>. 

 manner precisely similar to a pendulum 

 vibrating in .1 cycloid. It must therefore 

 agitate the air contiguous to it in the same 

 manner: and thus there is a particular 

 kind of agitation that the air can receive 

 and maintain, which is very interesting. 



Sir Issac Newton took up this question 

 as worthy of his notice ; and endeavoured 

 to ascertain with mathematical precision 

 the mechanism of this particular < 

 undulations, and gave us the principal 

 theorems concerning the undulations ot' 

 elastic fluids, which make the 47, &c. Fro 

 positions of Book II. of his Principles oi 

 Natural Philosophy. They have been 

 considered as giving the doctrines con- 

 cerning the propagation of sound. M 

 sounds, we ajl know, are conveyed to us 

 I)\ means of the air. In whatever manner 

 th'-y either float upon it, or an- propwlli o) 

 forward in it, certain it is, that, without 

 the vehicle of this or some other fluid, we 

 should have no sounds at all. Let the air 

 be exhausted from a receiver, and a bell 

 will emit no sound ; for, as the air conti 

 nues to grow less dcn.se, the sound dies' 

 in proportion, so that at last its 

 strongest vibrations arc almost totally si 

 lent. Thus air is a vehicle for sound 

 However, \\e must not, with some philo 

 gophers, assert, that it is the only vehicle. 

 that, if there were no air, weshoii' 



tor it is found, by 



experiment, that sounds are c<.- 

 through water with the same facility with 

 which they move through air. A bell rung 

 in water returns atone as distin 

 rung in air. 'lirsuas observed bv l)i 

 Derham, \\ ho also remarked, that the tone, 

 came a qnarter<lee|ier. It appea;- 



,-ieriments ofnaturaliys, that fishes 

 strong perception of - 



