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Proceeding a of the Poyal Society 
at the freezing point is only per cent, less than the maximum 
density, and is the same as the density at 8° C. ; and therefore 
when 3— per cent, is an admissible error on our ther-/^ 
mometric pressure, the density will be nearly enough 
constant with any temperature from 0° to 10° C. through- 
out the column. But on account of the first object 
mentioned above, the very top of the water-column must be 
kept with exceeding exactness at the freezing temperature. 
Fig. 2. 
In this instrument the “ thermometric substance ” is the water 
and vapour of water in the bulb, or more properly speaking the 
portions of water and vapour of water infinitely near their separating 
interface. The rest of the water is merely a means of measuring 
hydrostatically the fluid pressure at the interface. When the 
temperature is so high as to make the pressure too great to be 
conveniently measured by a water column, the hydrostatic measure- 
ment may be done, as shown in the second annexed drawing (fig. 2), 
by a mercury column in a glass tube, surrounded by a glass water 
jacket not shown in the drawing, to keep it very accurately at some 
