PRACTICAL ACOUSTICS.] 



UNDULATORY FORCES. ACOUSTICS. 



290 



but their real organs of sound are placed on the side of 

 the base of the abdomen, internal, and covered by a 

 cartilaginous plate, like a shutter, which is an appendage 

 of the under side of the meta-thorax or posterior thorax. 

 The cavity which incloses these instruments is divided 

 into two partitions by a scaly and triangular edge. Seen 

 from the under side of the body, each cell exhibits, 

 anteriorly, a white and folded membrane, and, in the 

 hollow part, a stretched-out, slender membrane, called 

 the mirror. If this part of the body be opened from 

 above on each side, there is seen another folded 

 membrane, which is moved by a very powerful muscle, 

 composed of a great number of straight and parallel 

 fibres, extending from the scaly ridge ; this membrane is 

 calied the iimbale. The muscles, by contracting and 

 relaxing with quickness, act upon the timhales, stretching 

 them out or bringing them into their natural state, 

 wheroby the sounds are produced, and which, even after 

 the deatli of the animal, may be repeated by moving 

 the parts over each other in the manner they act whilst 

 alive. The cicad;e occur chiefly in warmer countries of 

 the world. One species, the Cicada Aiiylica, the only 

 English species, is found in the New Forest. 

 It is a common belief that the buzzing of insects is 



Fig. 27. 



THORACIC HPIRACtr or Bl.fl-BOTTtl PIT. 

 (Muscn comitoria.) 



produced by the oscillations of their wings during flight. 

 This idea has been often called in question. John 

 Hunter found that insects emitted sounds after their 

 wings were cut off. More recently, it has been stated 

 that the sounds produced by many insects are the effect 

 of a rapid transmission of air through the thoracic air- 

 holes as they dash through space. Mr. Bishop has 

 observed a peculiar mechanism for this purpose in the 



Kg. 28. 



THORACIC IP1RACIR OP JU'MBI.E BRB. 



(Bombtis terreitris). 



blue-bottle fly, and humble bee. Figures 27 and 28, show 

 oceof the large thoracic spiracles in each of these insects 

 the Aftwca vomitoria and the Bombus terrestris. 



Having thus availed ourselves of Dr. Bushnan's 

 elaborate paper on animal sounds, we shall now proceed 

 to speak of other laws than those we have named ; and 

 also shall refer to various matters connected with daily 

 life, in which the principles we have investigated are 

 put into practical operation, or are more or less involved. 



PRACTICAL ACOUSTICS. 



UNDER the hpad of Practical Acoustics, we shall consider 

 those applications of the science which relate to the 

 construction of buildings specially devoted to concert* 



and public meetings ; including, at the same time, some 

 remarks on churches, and places of worship generally. 

 Other matters incidental to the subject will fall under 

 review. 



Speaking from personal experience, we do not hesitate 

 to affirm, that there are few buildings properly con- 

 structed on the principles of acoustics. In some places 

 of small size, a great deal more exertion is required on 

 the part of a public speaker, than in those capable of 

 containing a very large audience ; but which, by their 

 shape, are more fitted for the purposes for which they 

 have been erected. It is, however, a matter of con- 

 gratulation to all whose profession or position requires 

 of them to address an audience, that architects are, at the 

 present time, inclined to sacrifice their own views, with 

 respect to the appearance and ornamentation of an edifice, 

 to considerations of utility. It is a custom, in China, to 

 nail a baker by his ear to the doorway of his shop, if he 

 have been found guilty of selling an adulterated article ; 

 similarly, it might conduce to the comfort of both 

 speaker and audience, wore each architect compelled to 

 practise public speaking in a building of his own con- 

 struction, until such were fit for the purpose for which 

 it had been designed. 



Sound is a radial force ; that is, it proceeds from a 

 point, in straight lines in all directions, like the radii 

 or spokes of a wheel. Whatever object intercepts these 

 rays of sound, either reflects or absorbs them. If such 

 rays are emitted in the centre of a circular, or at the 

 foci of an elliptic or parabolic shaped hall or other 

 place, then they are reflected in exactly the same manner 

 as rays of heat and light ; and if such reflections take 

 place from any of the curved forms we have named, then 

 they are always inclined again to their source, and each 

 reflection harmonises with another. In a building so 

 constructed, we have the principles of acoustic science 

 attended to. In the majority of modern edifices, how- 

 ever, we do not find such principles consulted. An 

 eligible piece of ground has been purchased for a sum of 

 money, and the cost of a building on it is regulated by 

 the number of persons which can be crammed into it. 

 Each corner must be utilised ; peculiar modes of ingress, 

 egress, and ventilation are adopted ; and the only 

 question generally solved, is that of remuneration. 

 This evil, although great, may have a palliative in the 

 fact, that places of public amusement are notoriously a 

 bad speculation. But in the case of churches, <tc. , which, 

 generally speaking, are utterly independent of speculative 

 considerations, it is to be expected that some deference 

 would be paid to the rights of those who contribute to 

 the expenses, and to the manner in which the objects of 

 their erection are carried out. 



In most modern churches we find a cruciform structure. 

 The body of the building is of great length ; and, at some 

 part, recesses are formed, which tend to produce an 

 elegant appearance at the exterior. Now, as sound 

 radiates from its source, it follows the laws of all radial 

 forces namely, that its intensity decreases as the square 

 of the distance. Hence, persons sitting at the extremity 

 of many of our churches, are utterly unable to hear 

 sufficient of the discourse to understand its purport, 

 because the person addressing them is compelled to 

 modulate his voice, so that he may be heard distinctly 

 by the mass of his audience near to him, and that he 

 may avoid that undue straining of the voice which would 

 degenerate into "screaming," were he to attempt to 

 make himself heard in all parts. In many instances, 

 our churches have a length equal to three times their 

 breadth ; and as it is necessary that an audience should 

 bo ranged in curves, having successive and gradually 

 increased distance from the centre or focus, to be in 

 accordance with the principles of acoustics -it follows, 

 that all buildings of the class we have referred to, will 

 render the words of the speaker inaudible to many, or 

 he must exert himself to a most painful and harmful 

 degree, and to the annoyance of those near to him. 



There are other circumstances, independent of form, 

 which also affect the acoustic properties of public edifices. 

 For instance, in churches we find massive stuue pillar* 



