On the Determination of Heights, Sc. 287 
graduating thermometers, observers who have used the boil- 
ing point for the determination of heights, have always pre- 
ferred the ordinary tables which give the elasticity of steam 
in terms of its temperature, determined from experiments 
of quite a different kind from the boiling of water. 
Dr Dalton, indeed, has given a table from observation 
under the air-pump of the boiling point ;* and that table 
shews a manifest deviation from the elasticities and tempe- 
ratures of vapour determined by himself, and now generally 
accepted as the most accurate below 212°. In boiling, the 
temperature requires to be higher, under a given pressure, 
than the temperature of steam which has the same tension. 
Thus, comparing Dalton’s two tables— 
z ibe Tension of é 
Temperature, ae eri Vapour. Difference. 
212 30°0 30°0 
200 22°38 23°64 
190 18°6 19°00 
15:2 
it is exactly at the part of the scale where the difference is 
most practically important that it is most conspicuous, 
namely, between 190° and 212°. The method of observa- 
tion used by Dr Dalton, does not admit of any great accu- 
racy in observing the boiling points, and the numbers he 
has given are evidently only approximate. Still, from ob- 
servations made under naturally low pressures (the only ones 
worthy of much confidence in this case), I have found the 
same nonconformity of the theoretical tension of steam and 
the atmospheric pressure. 
In 1817, Archdeacon Wollaston described a thermometer 
destined particularly for the purpose of determining heights. t 
* Meteorological Essays, 2d edit. p. 127. 
+ Phil. Trans, vol, cxx. p. 183, 
