24 PROCEEDINGS OF THE AMERICAN ACADEMY 



to screw the spring higher upon to its support, E. This support con- 

 sists of a strip of brass fastened to a short pivot, which passes through 

 a hole in a plate attached to the rod F. The width of the strip of 

 brass, except at its lower end, is such that it lies freely within the coil 

 of the spring. At its lower end it has a projection on each side, which 

 give it a width a little greater than the external diameter of the coil. 

 These projections are notched to receive a single coil of the spring, 

 which is thus wholly supported from these notches, the part of the 

 spring above this point being entirely unaffected by the weight of the 

 apparatus below. In order to alter what may be called the " working 

 length " of the spring, it is only necessary to twist the spring A upon 

 the support E. The notches slide round upon the coils, and by thus 

 changing the point of support alter the length of that portion of the 

 spring which is affected by the weight attached to it. In the present 

 apparatus it is found that the shortening of the spring by a single coil 

 adjusts it for a fluid of 7°. 48 additional specific gravity. Thus if 

 blood (of Sp. Gr. = 1055) is the fluid used, as in measuring the 

 varying capacity of the blood-vessels of an organ, the spring has to be 

 shorter by 7.35 coils than in the case of distilled water. 



