90 O. N. Rood—Obtaining and measuring very high Vacua 
Arr. XVI.—On a method of obtaining and measuring very 
high Vacua with a modified form of Sprengel-pump ; by 
OapEeNn N. Roop, Professor of Physics in Columbia College. 
In the July number of this Journal] for 1880, I gave a short 
account of certain changes in the Sprengel-pump by means of 
which far better vacuua could be obtained than had been pre- 
viously possible. For example, the highest vacuum at that 
time known had been reached by Mr. Crooks, and was about 
aaaeeas While with my arrangement vacuua of zgp-pho-70T 
were easily reached. In a notice that appeared in “ Nature” 
for August, 1880, p. 875, it was stated that my improvements 
were not new, but had already been made in England four 
ears previously. I have been unable to obtain a printed ac- 
count of the English improvements, and am willing to assume 
that they are identical with my own ; but, on the other hand, as 
for four years no particular result seems to have followed their 
introduction in England, I am reluctantly forced to the conclu- 
sion that their inventor and his customers, for that period of 
time, have remained quite in ignorance of the proper mode 0 
utilizing them. Since then I have pushed the matter still far- 
ther, and have succeeded in obtaining with my apparatus 
vacuua as high as z55-y45-g9p) Without finding that the limit of 
its action had been reached. The pump is simple in construc- 
tion, inexpensive and, as I have proved by a large number 0 
experiments, certain in action aad easy of use: stopcocks and 
grease are dispensed with, and when the presence of a stopcock 
is really desirable its place is supplied by a movable column of 
mercury. 
cury and aor through a little watch-glass-shaped piece of 
sheet-iron, W, figure 1, which prevents the small air bubbles 
that creep upward along the tube from reaching its open end; 
the little cup is firmly cemented in its place. The flow of the 
mercury is regulated by the steel rod and cylinder OR, figure 1. 
The bottom of the steel cylinder is filled out with a circular 
piece of pure india-rubber, properly cemented; this soon fits 
itself to the use required ie answers admirably. The pres 
sure of the cylinder on the end of the tube is regulated by the 
lever 8, figure 1: this is attached to a circular board which 
