114 Mr. W. W. Jacques on the Effect of the Motion of the 



Every thing being in this condition and the sound-waves 

 having been mapped out, the doors and windows were thrown 

 open to the winter's air, and the registers were opened to 

 admit as many streams of air heated to nearly 100° C. What 

 is the effect ? Currents are rushing about the hall in every 

 direction (we have the conditions of the laboratory experi- 

 ment) ; the waves of sound are superimposed by numberless 

 reflections whose points of condensation do not coincide ; and 

 the phenomena of diffraction instantly disappear. This ex- 

 periment was many times repeated, and always with the same 

 results. 



Is is evident, then, that in order to procure the proper pro- 

 pagation of sound we must do away with these air-currents. 

 It must be remembered, however, that when large numbers 

 of persons are crowded into halls, the air within is usually 

 subjected to very considerable disturbances in order to obtain 

 even indifferent ventilation. How, then, shall we obtain the 

 desired ventilation and at the same time prevent the formation 

 of currents of air ? 



The solution of this problem seems to me to be given in the 

 third series of experiments, which were carried out in the 

 Baltimore Academy of Music, designed by Mr. J. Crawford 

 Neilson, architect, of that city. 



The ventilation of this house is so arranged as to prevent 

 largely the formation of air-currents of unequal density. Ac- 

 cording to a survey, made with thistle-balls and the anemo- 

 meter, of the space contained within the walls of this theatre, 

 the movement af the air is as follows : — 



The whole supply of fresh air is admitted at the back of the 

 stage, is there warmed, then crosses the stage horizontally, 

 passes through the proscenium, and then, somewhat diagonally 

 towards the roof, across the auditorium in one grand volume 

 and with gentle motion so as to almost entirely prevent the 

 formation of minor air-currents. It is exhausted partially by 

 an outlet in the roof and partly by numerous registers in the 

 ceilings of the galleries. From this central outlet and from 

 the large flues of the registers the air passes into the ventila- 

 ting-tower over the great chandelier, which supplies, in its 

 heat, a part of the motive power of the circulation. It is 

 further expelled from the tower by means of large valves so 

 contrived that, while they offer no obstacle to the egress of air, 

 they completely deny it entrance. The amount of air so passed 

 through the house is, as determined by a series of experiments, 

 about fifteen thousand feet per minute. This amount, suffi- 

 cient to ventilate the house, is just what seems to be required 

 to impress the proper movement on its atmosphere. That it 



