140 Le Roy’s Memoir on the best Method 
the centre of percussion of the balance, to produce: the de~ 
sired compensation: this is what is effected by a ball. of 
spirit of wine of about five lines and a half in diameter, 
which size may be varied according to the greater or less 
force of the liquor. Whence it is evident that we may. di- 
minish or increase the effect at pleasure to arrive at the pre- 
cise term of the compensation: Ist, by augmenting or di- 
minishing the size of the ball: 2dly, by placing the exterior 
branch further from, or nearer to, the centre of the balance; 
3dly, by using spirits of wine more or less, rectified. 
These thermometers, having a very smal] mass, produce 
their compensation at the same instant that the springs. are 
dilated or contracted: this is what cannot happen in all kinds 
of thermometers composed of metallic bars, which to be 
solid. must be made very strong with regard to the blade of 
the regulating spring. 
I use two thermometers instead of one, which, at first 
would’ appear to suffice; without this, in the different de- 
grees of heat and cold, the regulator would not preserve its 
equality of weight in all points of its circumference; which 
is an important thing, as I have explained, to prevent the 
effect of shocks and various motions. 
Before we terminate this article let us remove some diffi- 
culties that may be thrown in our way. 
Tt may at first be feared that the two liquids would mix 
by violent shocks; it is easy to be assured of the contrary 
by taking a similar thermometer, turning jt upside down, 
and shaking it. 
Perhaps it may also be apprehended that the spirit of wine 
would be compressed, more or less, according to the weight 
of the atmosphere, because I leave the inner branch open, 
i order that all the parts of the mercury and spirit of wine 
may tend with more force towards the ball: I will not deny 
that this will happen, but in so small a quantity that it must 
absolutely be neglected. By Mr. Canton’s experiments the 
whole weight of the atmosphere only compresses the spirit 
of wine the +,45syath part 5. therefore the variations of these 
weights, which hardly amount to +;th, would only form in 
24 hours the variation of four millionth and a half of 5’ or 
300”; 
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