with a modified form of Sprengel-pump. 97 
area second time measured. Assuming the pressure to remain 
constant, or that the volumes are reduced to the same pressure, 
v=the original volume ; 
v'=the final vo : 
V'=volume of air introduced by the first entry of the mercury; 
V=corrected volume ; then 
_ vv 
nite oe 
V=v0— enet 
It will be noticed that it is assumed in this formula that the 
same amount of air is introduced into the gauge in the acts of 
entry and exit; in the act of entering in point of fact more fresh 
mercury is exposed to the action of the vacuum than in the act 
exit, which might possibly make the true gauge-correction rather 
_larger than that given by the formula. It has been found that 
when the pump is in constant use the gauge-correction gradually 
diminishes from day to day: in other words, the air is gradually 
pumped out of the gauge-mercury. Thus on December 21st, the 
amount of air entering with the mercury corresponded to an 
exhaustion o 
1 
nae enh Dee, 21st. 
27 308 805 
ee Tae Dec, 29th. 
a age. Jan. 15th. 
83 943 ee oe 
on Sa OG 22 arene 1a, 
1 | 
396 757 400-77 7--E oP 9th. 
1 nr 
ga eon bog tO 
] 
388 200 000 a ae a March 7th. 
‘That this diminution is not due to the air being gradually 
withdrawn from the walls of the gauge or from the gauge-tube, is 
shown by the fact that during its progress the pump was 
several times taken to pieces, and the portions in question 
exposed to the atmosphere without affecting the nature or 
extent of the change that was going on. I also made one 
experiment which proves that the gauge-correction does not 
