308 PROCEEDINGS OF THE AMERICAN ACADEMY 



suspension, over the mouth of the trunk, is such that the marble hangs 

 as nearly as possible in its centre. The handle is made to revolve ac- 

 curately once a second, and the deflection of the marble from the point 

 of rest, under the influence of the blast thus produced, observed. The 

 marble is then protected from the blast, and the effect of the blast upon 

 the thread alone observed and deducted from the first result. To as- 

 certain the value of this deflection, the following method is adopted. 

 Into a large cylindrical vessel, filled with water, a pipe, an inch in di- 

 ameter and bent into the form of an inverted syphon, is so placed, that, 

 while one of its branches rises in the centre of the vessel, an inch 

 above the surface of the water, the other branch rises along the side 

 of the vessel, over which it is bent nearly horizontally. Another and 

 similar vessel 15.5 inches in diameter at the top, 14 inches at the bot- 

 tom, and 8.25 inches in depth, is inverted upon the surface of the 

 water in the first. By pressing down this second vessel the contained 

 air is made to issue from the open extremity of the pipe ; and as the 

 areas of the vessel and pipe are both known, we have but to note 

 the time required to empty the second vessel to learn the velocity of the 

 escaping air. The marble is now suspended by the same thread ; the 

 point of suspension being so situated that the marble falls against the 

 mouth of the pipe, and would, if allowed to move freely, hang as far 

 within it as the marble, deducting the effect upon the thread, was de- 

 flected by the blast. The second vessel is now depressed with such 

 velocity that the marble is just made to swing clear of the mouth of the 

 pipe, by which its deflection becomes precisely that produced by the 

 blast which is to be measured. 



"In the case under consideration, the deflection of the thread and 

 marble together was 2.5 inches ; that dependent upon the thread alone, 

 .95 inch. The time occupied in depressing the vessel until it rested 

 upon the top of the inverted syphon, in several successive experiments, 

 was 12.25 seconds. The contained air was compressed .25 inch to 

 produce this velocity, and, as the pipe rose 1 inch above the surface of 

 the water, 1.25 inches were deducted from the depth of the vessel, leav- 

 ing an available depth of 7 inches. The mean diameter, that at the 

 top being 15.5 inches, and at the bottom 14 inches, is 14.75 inches. 

 As the areas of circles are to each other as the squares of their diam- 

 eters, we have these areas in the proportion of 217.56 to 1. This 

 number multiplied by the depth in inches, 7, gives the whole expendi- 

 ture in 12.25 seconds, the time required to empty the vessel ; from which 



