75 
on the Cohesion of Fluids, 
the thickness at which a quantity of mercury will stand when 
spread out on glass, supposing the angle of contact still 140°, 
are found, by taking the proportion of the sines of 20° and of 
70° to the sine of 45 0 , and are therefore .0484 and .1330 
respectively. If, instead of glass, we employed any surface 
capable of being wetted by mercury, the height of elevation 
would be .141, and this is the limit of the thickness of a wide 
surface of mercury supported by a substance wholly incapable 
of attracting it. Now the hydrostatic pressure of a column of 
mercury .0484 in thickness on a disc of one inch diameter 
would be 131 grains ; to this the surrounding elevation of the 
fluid will add about 11 grains for each inch of the circum- 
ference, with some deduction for the effect of the contrary 
curvature of the horizontal section, tending to diminish the 
height; and the apparent cohesion thus exhibited will be 
about 160 grains, which is a little more than four times as great 
as the apparent cohesion of glass and water. With a disc 1 1 
lines in diameter Mr. Dutour found it 194. French grains, 
which is equivalent to 152 English grains, instead of ibo, for 
an inch, a result which is sufficient to confirm the principles of 
the calculation. The depth of a quantity of mercury standing 
on glass I have found by actual observation, to agree precisely 
with this calculation. Segner says that the depth was .1358, 
both on glass and on paper : the difference is very trifling, but 
this measure is somewhat too great for glass, and too small 
for paper, since it appears from Dutour's experiments, that 
the attraction of paper to mercury is extremely weak. 
If a disc of a substance capable of being wetted by mercury, 
an inch in diameter, were raised from its surface in a position 
perfectly horizontal, the apparent cohesion should be 381 
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