344 Dr. Ure's new experimental researches 
mercury in a slender stream into the open leg of the syphon 
D. The level ring below is then carefully adjusted. 
From the mode of conducting my experiments, there 
remained always a quantity of liquid in contact with the 
vapour, a circumstance essential to accuracy in this research. 
Suppose the temperature of the water or the oil in A to be 
32 0 F., as denoted by a delicate thermometer, or by the 
liquefaction of ice ; communicate heat to the cylinder A, by 
means of two argand flames, playing gently on its shoulder 
at each side. When the thermometer indicates 42 0 , modify 
the flames or remove them, so as to maintain an uniform 
temperature for a few minutes. A film or line of light will 
now be perceived between the mercury and the ring at /, as 
is seen under the vernier of a mountain barometer when it is 
raised a few feet off the ground. Were the tube at / and L 
of equal area, or were the relation of the areas experimentally 
determined, then the rise of the quicksilver above L would 
be one half, or a known submultiple of the total depression, 
equivalent to the additional elasticity of the vapour at 42° 
above that at 32 0 . Since the depressions, however, for 30 or 
40 degrees in this part of the scale are exceedingly small, 
one half of the quantity can scarcely be ascertained with 
suitable precision, even after taking the above precautions. 
And besides, the other sources of error, or at least embarrass- 
ment, from the inequalities of the tube, and from the lengthen- 
ing space occupied by the vapour, as the temperature ascends, 
render this method of reduction very ineligible. 
By the other plan we avoid all these evils. For whatever 
additional elasticity we communicate to the vapour above /, 
it will be faithfully represented and measured, by the 
