96 O. N. Kood—Obtaining and measuring very high Vacua 
of these to each other. This I did quite elaborately, and 
proved that such constant errors as exist, are small, compared 
with inevitable accidental errors, as for example that there was 
no measurable correction for capillarity, that the calculated 
f the ‘‘meniscus” was correct, ete. It is essential in 
making a measurement that the temperature of the room should 
change as little as possible, and that the temperature of the 
mercury in the cylinder should be at least nearly that of the 
air near the gauge-sphere. The computation is made as follows: 
n=height of the cylinder enclosing the air ; 
e=a factor which multiplied by » converts it into cubic 
millimeters ; 
S=cubic contents of the meniscus ; 
d=ditference of level between A and B, fig. 4; 
=the pressure the air is under ; 
N=the cubic contents of the guage in millimeters ; 
t= pore expressing the degree of exhaustion obtained : | 
then 
A leer 
ne—§ 
It will be noticed that the measurements are independent of 
the actual height of the. barometer, and if several readings are 
taken continuously, the result will not be sensibly affected by 
a simultaneous change of the barometer. Almost all the read- 
ings were taken at a temperature of about 20° C., and in the 
present state of the work corrections for temperature may be 
considered a superfluous refinement. 
auge correction.—It is necessary to apply to the results thus 
obtained a correction which becomes very important when 
it is necessary to subtract the volume of this air from nc. By 
a series of experiments I ascertained that the amount of air 
introduced by the mercury in the acts of entering and leaving 
the gauge was sensibly constant for six of these single operations 
(or for three of these double operations), when they follow 
each other immediately. The correction accordingly is made 
as follows: the vacuum is first measured as described above, 
then by withdrawing all the boxes except the lowest, the mer- 
cury is allowed to fall so as nearly to empty the gauge; it is then 
made again to fill the gauge, and these operations are repeat 
until they amount in all to six; finally the volume and pressure 
