with a modified form of Sprengel-pump. 91 
again is firmly fastened over the open end of the bell-glass. It 
will be noticed that on turning the milled head 8, the motion 
of the steel cylinder is not directly vertical, but that it tends to 
describe a circle with ¢ as a center; the necessary play of the 
cylinder is however so small, that practically the experimenter » 
does not become aware of this theoretical defect, so that the 
arrangement really gives entire satisfaction, and after it has 
been in use for a few days accurately controls the flow of the 
mercury. The glass cylinder is held in position, but not sup- 
ported, by two wooden adjustable clamps aa, figure 2. The 
weight of the cylinder and mercury is supported by a shelf, 8, 
figure 2, on which rests the cork of the cylinder; in this way 
all danger of a very disagreeable accident is avoided. 
Vacuum-bulb.—Leaving the reservoir, the mercury enters the 
vacuum-bulb B, figure 2, where it parts with most of its air and 
moisture; this bulb also serves to catch the air that creeps 
into the pump from the reservoir, even when there is no flow 
of mercury ; its diameter is 27™™, The shape and inclination 
of the tube attached to this bulb is by no means a matter of 
indifference; accordingly figure 3 is a separate drawing of it; 
the tube should be so bent that a horizontal line drawn from 
po e 
the tube EC should be 150™, that of the tube ED 45™; the 
bore of this tube is about the same as that of the fall-tube, 
