ACOUSTICS. 



perhaps supply the place of foreign spices; 

 and 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. 



ACOTYLEDONES, in botany ,plants so 

 called because their seeds are not furnish- 

 ed with lobes, and of course put forth no 

 seminal leaves. All mosses are of this 

 kind. See COTYLEDOXES. 



ACOUSTICS, in physics, is that science 

 which instructs us in the nature of sound. 

 ft is divided by some writers into diacous- 

 tics, which explains the properties of those 

 sounds that come distinctly from the so- 

 norous body to the ear ; and catacoustics, 

 which treats of reflected sounds ; but this 

 distinction is not necessary. In the infan- 

 cy of philosophy, sound was held to be a 

 separate existence ; it was conceived to 

 be wafted through the air to our organs 

 of hearing, which it was supposed to affect 

 in a manner resembling that in which our 

 nostrils are affected when they give us the 

 sensation of smell. Yet, even in those 

 early years of science, there were some, 

 and in particular, the celebrated founder 

 of the Stoic school, who held that sound, 

 that is, the cause of sound, was only the 

 particular motion of external gross matter, 

 propagated to the ear, and there produc- 

 ing that agitation of the organ, by which 

 the soul is immediately affected with the 

 sensation of sound. Zeno says, " Hearing 

 is produced by the air which intervenes 

 between the thing sounding and the ear. 

 The air is agitated in a spherical form, and 

 moves off in waves, and falls on the ear, 

 in the same manner as water undulates in 

 eirclesVhen a stone has been thrown into 

 it." The ancients were not remarkable 

 for precision, either of conception or ar- 

 gument, in their discussions, and they 

 were contented with a general and vague 

 view of things. Some followed the opinion 

 of Zeno, without any farther attempts to 

 give a distinct conception of the explana- 

 tion, or to compare 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 

 gave the first opportunity of deciding, by 

 experiment, whether the elastic undula- 

 tions of air were the causes of sound; and 

 the trial fully established the point; for a 

 bell rung in vacuo gave no sound, and one 

 rung in condensed air gave a very loud 

 one. It was therefore received as a doc- 

 trine in general physics, that air was the 

 vehicle of sound. The celebrated Galileo, 

 the, parent of mathematical philosophy, 



discovered the nature of that connection 

 between the lengths of musical chords and 

 the notes which they produced, which had 

 been observed by Pythagoras, or learned 

 by him in his travels in the East, and which 

 he made the foundation of a refined and 

 beautiful science, the theory of music. 

 Galileo shewed, that the real connection 

 subsisted between the tones and the vibra- 

 tions of these chords, and that their dif- 

 ferent degrees of acuteness corresponded 

 to the different frequency of their vibra- 

 tions. The very elementary and 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 a 

 manner precisely similar to a pendulum, 

 vibrating in a 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 Isaac Newton took up this question 

 as worthy of his notice ; and endeavoured 

 to ascertain with mathematical precision 

 the mechanism of this particular class of 

 undulations, and gave us the principal 

 theorems concerning the undulations ot' 

 elastic fluids, which make the 47, &c. Pro- 

 positions of Book II. of his Principles of 

 Natural Philosophy. They have been 

 considered as giving the doctrines con- 

 cerning the propagation of sound. Most 

 sounds, we all know, are conveyed to us 

 by means of the air. In whatever manner 

 they either float upon it, or are propelled 

 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 dense, the sound dies 

 away in proportion, so that at last its 

 strongest vibrations are almost totally si- 

 lent. Thus air is a vehicle for sounds 

 However, we must not, with some philo> 

 sophers, assert, that it is the only vehicle; 

 that, if there were no air, we should have 

 no sounds whatsoever ; for it is found, by 

 experiment, that sounds are conveyed 

 through water with the same facility with 

 which they move through air. A bell run< 

 in water returns a tone as distinct as if 

 rung in air. This was observed by Dr, 

 Derham. who also remarked, that the tone 

 came a quarter deeper. It appears, from 

 the experiments of naturalists, that fishes 

 have a strong perception of sounds, even 



