436 
Proceedings of the Royal Society 
cular temperature 0° C., and a stream of water as nearly as may be 
of atmospheric temperature will be the easiest as well as the most 
accurate way of keeping the mercury at a 
definite temperature. As the pressure of mer- 
cury steam is at all ordinary atmospheric 
temperatures quite imperceptible to the hydro- 
static test when mercury itself is the balancing 
liquid, that which was the chief reason for 
fixing the temperature at the interface between 
liquid and vapour at the top of the pressure- 
measuring column when the balancing liquid 
was water, has no weight in the present case ; 
but, on the other hand, a much more precise 
definiteness than the ten degrees latitude 
allowed in the former case for the temperature 
of the main length of the manometric column 
Fig. 3. 
is now necessary. In fact, a change of temperature of 2*2° in 
mercury at any atmospheric temperature produces about the same 
proportionate change of density as is produced in water by a 
change of temperature from 0° to 10°, that is to say, about 
per cent. ; but there is no difficulty in keeping, by means of a 
water jacket, the mercury column constant to some definite tem- 
perature within a vastly smaller margin of error than 2*2°, 
especially if we choose for the definite temperature something 
near the atmospheric temperature at the time, or the temperature 
of whatever abundant water supply may be available. If the 
glass tube for the pressure-measuring mercury column be 838 
centimeters long, the simple mercury-steam thermometer may be 
used up to 520° C., the highest temperature reached by Eegnault 
in his experiments on mercury-steam. By using an iron bulb 
and tube for the part of the thermometer exposed to the high 
