132 AMERICAN ASSOCIATION FOR THE 



to appeal to experiment and observation, not only to assist in deducing consequences, 

 but also to verify those which have been arrived at by logical deduction. Further- 

 more, though we may know the manner in which a cause acts to produce a given 

 effect, yet in all cases we are obliged to ascertain the measure of effect under given 

 conditions. 



The science of acoustics as applied to buildings, perhaps more than any other, 

 requires this union of scientific principles with experimental deductions. While 

 on the one hand the simple deductions from the established principles of acoustics 

 would be unsafe from a want of knowledge of the constants which enter into our 

 formula, on the other hand empirical data alone are in this case entirely at fault, 

 and of this any person may be convinced who will examine the several works 

 written on acoustics by those who are deemed practical men. 



Sound is a motion of matter capable of affecting the ear with a sensation pecu- 

 liar to that organ. It is not in all cases simply a motion of the air, for there are 

 many sounds in which the air is not concerned. For example, the impulses which 

 are conveyed along a rod of wood from a tuning fork to the teeth. When a sound 

 is produced by a single impulse, or an approximation to a single impulse, it is called a 

 noise — when a series of impulses, a continued sound, &c. ; if the impulses are 

 equ?l in duration among themselves, a musical sound. This has been illustrated 

 by a quill striking against the teeth of a wheel. A single impulse from one tooth 

 is a noise, from a series of teeth in succession a continued sound, and if all the teeth 

 are at equal distances, and the velocity of the wheel is uniform, then a musical note 

 is the result. Each of these sounds is produced by the human voice, though they 

 apparently run into each other. Usually, however, in speaking, a series of irregu- 

 lar sounds of short duration are omitted — each syllable of a word constitutes a sep- 

 arate sound of appreciable duration, and each compound word and sentence an as- 

 semblage of such sounds. It is astonishing that in listening to a discourse the ear 

 can receive so many impressions in the course of a second, and that the mind ean 

 take cognizance of and conquer them. 



That a certain force of impulse, and a certain time for its continuance are ne- 

 cessary to produce an audible impression on the ear is evident; but it may be 

 doubted whether the impression of a sound on this organ is retained appreciably long, 

 er than the continuance of the impulse itself. Certainly not longer than the l-10th 

 of a second. If this were the case, it is difficult to conceive why articulate dis- 

 course which so pre-eminently distinguishes man from the lower animals, should 

 not fill the ear with a monotonous hum ; but whether the ear continues to vibrate, 

 or whether the impression remains a certain time on the sensorium, it is certain 

 that no sound is ever entirely instantaneous, or the result of a single impression, 

 particularly in inclosed spaces. Every impulse must give rise to a forward, and 

 afterwards to a backward motion of the atom. The impulse is net only communica- 

 ted to the ear but to all bodies around, which, in turn, themselves become centres 

 of reflected impulses. 



Sound, from a single explosion in air equally elastic on all sides, tends to expand 

 equally in every direction ; but when the impulse is given to the air in a single di- 

 rection, through an expansion taken place on all sides, it is much more intense in 

 the line of the impulse. For example, the impulse of a single explosion, like that 

 of the detonation of a bubble of oxygen, is propagated equally in all directions; 

 while the discharge of a cannon, while heard on every side, is much louder in the 

 direction of the axis; so, also, a person speaking is heard much more distinctly in 



