of Edinburgh, Session 1879 - 80 . 
437 
-30 
-25 
- 20 ' 
10 ° 
15"4 
-10 
- 15 ° 
■ - 20 ° 
- 25 ° 
- 30 ° 
temperature, and for the lower part of the measuring column to 
within a few metres of its top, with glass for the upper part to 
allow the mercury to he seen, a mercury-steam-pressure ther- 
mometer can wdth great ease he made which shall 
he applicable for temperatures giving pressures up 
to as many atmospheres as can he measured hy 
the vertical height available. The apparatus may 
of course he simplified hy dispensing with the 
Torricellian vacuum at the upper end of the tube, 
and opening the tube to the atmosphere, when the 
steam-pressure to be measured is so great that a 
rough and easy barometer observation gives with 
sufficient accuracy the air-pressure at the top of 
the measuring column. The easiest, and not ne- 
cessarily in practice the least accurate, way of 
measuring very high pressures of mercury-steam 
will be by enclosing some air above the cool, 
pressure-measuring column of mercury, and so 
making it into a compressed-air pressure-gauge, it 
being understood that the law of compression of 
the air under the pressures for which it is to be 
used in the gauge is known by accurate inde- 
pendent experiments such as those of Regnault on 
the compressibility of air and other gases. 
The water-steam thermometer may be used, but 
somewhat precariously, for temperatures below the 
freezing-point, because water, especially when en- 
closed and protected as the portion of it in the 
bulb of our thermometer is, may be cooled many 
degrees below its freezing-point without becoming 
frozen ; but, not to speak of the uncertainty or 
instability of this peculiar condition of water, the 
instrument would be unsatisfactory on account of insufficient 
thermometric sensibility for temperatures more than two or three 
degrees below the freezing-point. Hence, to make a steam 
thermometer for such temperatures some other substance than water 
should be taken, and none seems better adapted for the purpose 
than sulphurous acid, which, in the apparatus represented with 
Fig. 4. 
