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the fea at great depths, fome alteration would he 
necelfary in the conllrudtion of them, principally 
upon account of the great prelfure of the water ; 
the ill effedt of which can, I believe, be prevented 
no other way, than by leaving the tube open. For 
if the thermometer was made ftrong enough to refill 
the prefliire without breaking, yet it would be im- 
poflible to be fure, that the figure of the glafs would 
not thereby be altered, which Ihould make the ex- 
periment uncertain. 
The inllrument for finding the greatell heat might 
be made jull like that of fig. i. only leaving the top 
open. It is to be filled with mercury only, as is 
alfo the lower part of the ball at top, but not near 
fo high as the end of the capillary tube. The upper 
part of that ball, being left open, will in a great 
ineafure be filled with the fea-water, which will be 
forced into it by the prelfure. 
If this inllrument (the tube being quite full of 
mercury) is plunged into any part of the fea, where 
the heat is greater than that of the air above, part of 
the mercury will be driven out of the tube ; and, 
upon bringing it into a colder place, the fea-water or 
air in the ball will enter into the tube, and will fill 
the fpace left by the mercury. 
As this thermometer does not Ihow the common 
degrees of heat, it mull be placed in a vefiel of 
water with another thermometer, and the fcale of 
degrees at top will fhew how much the heat it has 
been expofed to is greater than that of the water in 
the velfel. 
The fea-water getting into the glafs will corrode 
the mercury, and thereby foul the glafs j which 
R r 2 will 
