92 O. N. Rood—Obtaining and measuring very high Vacua 
Fall-tube and bends.—The bore of the fall-tube in the pump 
now used by me is 1°78™™ ; its length above the bends (U, figure 
2) is 310™; below the bends the length is 815™. ‘The bends 
constitute a fluid valve that prevents the air from returning into 
the pump; beside this, the play of the mercury in them greatly 
facilitates the passage of the air downward. The top of the 
mercury column representing the existing barometric pressure 
should be about 25™" below the bends when the pump is 1n 
action. This is easily regulated by an adjustable shelf, which 
is also employed to the bends with mercury when a meas- 
urement is taken or when the pump is at rest. On the shelf 
perforation can be easily made and shaped in a few minutes. 
By revolving the little bent tube through 180° the flow of the 
mercury can be temporarily suspended when it is desirable to 
change the vessel that catches it. ; 
Gauge.—For the purpose of measuring the vacua I have 
used an arrangement similar to McLeod’s gauge, fig. 4; it has, 
however, some peculiarities. The tube destined to contain the 
compressed air has a diameter of 1:35™, as ascertained by @ 
compound microscope; it is not fused at its upper extremity, 
but closed by a fine glass rod that fits into it as accurately as 
may be, the end of the rod being ground flat and true. This 
rod is introduced into the tube, and while the latter is gently 
heated a very small portion of the cement described below 1s 
allowed to enter by capillary attraction, but not to extend be- 
yond the end of the rod, the operation being watched by a lens. 
The rod is used for the purpose of obtaining the compressed 
air in the form of a cylinder and also to allow cleansing of the 
tube when necessary. The capacity of the gauge-sphere was 
obtained by filling it with mercury ; its external diameter was 
sixty millimeters; for measuring very high vacua this is some- 
what small and makes the probable errors rather large; 
would advise the use of a gauge-sphere of about twice as great 
capacity. The tube CB, figure 4, has the same bore as the 
measuring tube in order to avoid corrections for capillarity- 
The tube of the gauge CD is not connected with an india-rub- 
ber tube, as is usual, but dips into mercury contained in a 
cylinder 340™" high, 58™" in diameter, which can be raise 
and lowered at pleasure. This is best accomplished by the use 
of a set of boxes of various thicknesses, made for the purpose 
and supplemented by several sheets of cardboard and even ° 
writing-paper. These have been found to answer well and 
enable the experimenter to graduate with a nicety the pressure 
