70 PROCEEDINGS OF THE AMERICAN ACADEMY. 



above is shown in dotted lines. In many respects the constant tem- 

 perature room offered admirable conditions for the experiment. Its 

 position in the centre of the building and its depth underground 

 made it comparatively free from outside disturbing noises, — so much 

 so that it was possible to experiment in this room in the earlier parts 

 of the evening, although not, of course, when any one else was at work 

 in the building. While it possesses these advantages, its arched ceil- 

 ing, by placing it in the category of special cases, makes extra pre- 

 caution necessary. Fortunately, at the beginning of the experiment 

 the walls were unpainted. Under these conditions its coefficient of 

 absorption for different notes was determined. It was then painted 

 with an oil paint, two coats, and its coefficient of absorption re- 

 determined. The two curves are shown in Figure 4. The upper 

 curve is for the unpainted brick ; the lower curve is that obtained 

 after the walls were painted. The difference between the two curves 

 would, if plotted alone, be the curve of absorption due to the poros- 

 ity of the brick. It may seem, perhaps, that the paint in covering 

 the bare brick wall made a smoother surface, and the difference be- 

 tween the two results might be due in part to less surface friction. Of 

 course this is a factor, but that it is an exceedingly small factor will be 

 shown later in the discussion of the results on the absorption of sound 

 by other bodies. The absorption of the sound after the walls are 

 painted is, of course, due to the yielding of the walls under the vibra- 

 tion, to the sound actually transmitted bodily by the walls, and to the 

 absorption in the process of transmission. It is necessary to call atten- 

 tion to the fact that the vertical ordinates are here magnified tenfold 

 over the ordinates shown in the last curve. 



The next experiment was on the determination of the absorption of 

 sound by wood sheathing. It is not an easy matter to find conditions 

 suitable for this experiment. The room in which the absorption by 

 wood sheathing was determined in the earlier experiments was not 

 available for these. It was available then only because the building 

 was new and empty. When these more elaborate experiments were 

 under way the room had become occupied, and in a manner that did 

 not admit of its being cleared. Quite a little searching in the neigh- 

 borhood of Boston failed to discover an entirely suitable room. The best 

 one available adjoined a night lunch room. The night lunch was bought 

 out for a couple of nights, and the experiment was tried. The work of 

 both nights was much disturbed. The traffic past the building did not 

 stop until nearly two o'clock, and began again about four. The interest 

 of those passing by on foot throughout the night, and the necessity of 

 repeated explanations to the police, greatly interfered with the work. 



