258 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



allow the compressing fluid to flow freely from one end of the cylinder 

 to the other. The change of length of the rod is obtained by keeping 

 one end of the rod always fixed opposite the same part of the cylinder, 

 and measuring the relative displacement of the other end, which is 

 free. The fixed end of the rod is kept so by the action of the spring at 

 E, which keeps the rod pressed against the stop at D. This stop D 

 is kept immovable by the spring at C, which keeps D pressed against 

 the shoulder B. This spring C is very much stiffer than the spring E, 

 and is kept permanently compressed by the pump connections (not 



Figure 2. Enlarged view of the brass ring, etc., of Figure 1. The dis- 

 placement of the ring is measured by measuring the distance between the 

 scratches at L and M on the rod and the ring respectively. 



shown) which are screwed into the end A, and keep the ring J fast 

 in the position shown. This method of securing the invariable position 

 of the stop seemed preferable to any plug arrangement screwed fast 

 into the cylinder, for the latter might shift slightly, owing to the 

 change produced by the pressure in the dimensions of the thread. 



The shift of the free end of the rod relatively to the cylinder was 

 obtained by measuring the displacement on the brass ring F, which 

 is pushed back by the shoulder G. An enlarged view of the ring is 

 shown in Figure 2. The brass ring F is split so as to slide without 

 too great friction on the end of the rod, which is turned down to 

 about A in. (8 mm.). There is a fine scratch on the ring at M, and 

 also a scratch on the corresponding ledge L of the rod. The ring 

 and rod are turned in the lathe so that these two scratches are at the 

 same radial distance from the axis of the rod, thus enabling both 

 scratches to be in focus simultaneously under a high power microscope. 

 The effect of an application of pressure is to shorten the rod, pushing 



