130 TRANSACTIONS OF THE [MAR. 4, 
superimposed, they present the aspect of an ordinary geological 
map, and can then be turned down separately, so as to show the 
entire area actually occupied by each formation, although, in 
fact, covered more or less by later ones. He explained the con- 
struction and use of the map, and the various points of geologi- 
cal structure which it can be made to elucidate. 
The apparatus was much admired by the members, and was 
discussed by the PRESIDENT, PRoF. MARTIN, and others. 
— 
March 4, 1889. 
REGULAR BusINESS MEERTING. 
The Academy met in the Chemical Lecture-room of the 
School of Mines building, for the sake of certain facilities in the 
use of apparatus for experimental illustration of the lecture. 
The President, Dk. NEWBERRY, in the Chair. 
A large audience was present. 
The Report of the Council was presented, containing only a 
few minor items of business, which were unanimously approved. 
The third Public Lecture of the season’s course was then de- 
livered by Pror. W. LE ContE STEVENS, of Brooklyn, upon 
THE DIFFRACTION OF SOUND. 
(Abstract. ) 
The lecturer gave an introductory sketch of the development of 
the wave theory in its application to sound and light, as brought 
out by Lord Bacon, Galileo, Newton, Hooke, Huygens, Young, 
Arago, and Fresnel. With the experiments of these physicists 
he contrasted the speculations of a small but persistent modern 
school, who have been endeavoring to overturn the wave theory 
and to substitute the so-called ‘‘substantial” theory. If sound 
or light can be propagated by the ‘‘emanation of an immaterial 
substance,” its immateriality removes the consideration of it from 
the domain of physical science, which is concerned exclusively 
with the Jaws of matter and motion. No answer from any 
