44-5 Mr. cavallo’s Defcription of 
When the hole in the plate of the pump is flopped up by 
means of a fcrew and leather, and the inflrument is worked for 
about three or four minutes, the quickfilver in the fmall tube 
of the gage falls fo low as to be even with the quickfilver in 
the outfide tube, which (hews as if the air were entirely ex- 
haufted from the infide of the pump ; but as it is difficult to 
judge whether the two furfaces of the mercury in the infide 
and outfide tube are quite on the fame level, and even if they 
were exactly fo, there could always be fufpecled that a little 
air may be lodged within the gage, notwithflanding that fede- 
ral gages of this fort have been ufed, in fome of which the 
mercury had been accurately boiled; we muff, therefore, have 
recourfe to other gages, in order to determine the exhaufling 
power of this pump with more precifion. Accordingly, the 
pear- gage and long barometer-gage were tried, the effects of 
which will be related in the fequel. 
If, inflead of flopping up the hole in the plate of the pump, 
a glafs receiver is laid upon it, and the pump is worked, the 
gage will alfo come fo low as when no receiver is put upon it ; 
but it mufl be remarked, that after exhaufling and leaving the 
inflrument in that flate, when no receiver is upon it, the quick- 
filver in the gage will be rifing for about one hour, fo as to 
afcend one- tenth, or at mofl one-fifth of an inch above the 
furface of the quickfilver in the outward tube, and then it re- 
mains flationary ; whereas if the experiment be repeated when 
the receiver is upon the plate, the degree of exhauflion remains 
unaltered, the mercury not rifing at all in the fmall tube. This 
rifing of the quickfilver in the firfl cafe feems.to be occafioned 
by fome fmall quantity of elaflic fluid, that is yielded by the 
oil contained between the parts of the machine; for this quan- 
tity of elaflic fluid can occafion a fenfible difference when the 
exhaufled 
