OF ARTS AND SCIENCES : OCTOBER 8, 1867. 418 



it as such. In fact, I have no belief that more than 70 pounds of 

 powder should be assigned as the service charge of the Armstrong 

 13*3-inch gun ; as no gun can be trusted for long-continued firing with 

 more than y^^g- of the largest charge of powder which it may have 

 withstood, and no cast-iron gun with so much as this. 



I have not the data necessary to determine accurately the velocity, and 

 consequently the force, which this reduction of the charge of powder 

 must make in the shot ; but if we take the force of the shot in the 

 direct ratio of the weight of the charge of powder, we shall have 261, 

 instead of 372-8, as representing the " number of pounds of shot raised 

 one foot by each pound of metal in the gun," as these numbers are in 

 the ratio of 70 to 100. 



I am not able to state what has constituted the greatest charge of 

 powder borne by Armstrong's gun of 12 tons, carrying a shot of 300 

 pounds ; but reducing the charge of 60 pounds, as given by me in the 

 ratio of 70 to 100, we have a charge of 42 instead of 60 pounds of 

 powder, and a consequent reduction of the force of the gun from 392 

 "foot pounds" to 273 "foot pounds." 



I have thought it the more necessary to make this correction, as in 

 a computation of the force of the Dahlgren and Rodman guns, given in 

 the same paper to which this is a correction, the quantity of powder 

 then understood by me from all that had been published by govern- 

 ment authority as constituting a service charge was taken as one of 

 the factors in assigning the measure of the force to those guns. It is 

 now claimed, however, to have been discovered that the Rodman gun is 

 capable of withstanding much larger charges of powder than were 

 authorized to be used when my paper was communicated to the Acad- 

 emy. 



Professor Lovering made the following remarks on the 

 Optical Method of studying Sound, illustrating the subject 

 by many experiments : — 



When the science of Acoustics is studied by means of the ear ex- 

 clusively, we judge of the process simply by the result, that is, by the 

 sensation. The optical method of investigation often gives us an in- 

 sight into the process itself. Sound begins with a stationary vibration 

 in the sonorous body ; it is propagated by a progressive undulation ; 

 and it ends, physically and mechanically considered, in a vibration of 

 some one of the three thousand nervous filaments discovered by Corti in 



