288 Professor Forbes on the Determination of Heights 
But he seems not to have been aware of the progress which 
the subject had already made in the hands of Deluc and De 
Saussure. The latter used a thermometer indicating 7>5> 
of a degree of Reaumur. Wollaston’s instrument, though a 
neat laboratory one, has almost every fault which a travel- 
ling instrument can have, excepting only its small dimen- 
sions, to which everything is sacrificed. It is apt to break, 
and still more apt to be deranged, the contrivance for ex- 
tending the scale being excessively incommodious ; finally, 
it is impossible to use it in windy weather, and its indica- 
tions are in an arbitrary scale. Nor was the method of cal- 
culating the heights more happy. At first he contented 
himself with assuming the progression of height to be pro- 
portional to the fall of the boiling point, near 212°;* but 
he afterwardst extended his calculation from Dr Ure’s table 
of tensions of vapour, expressly stating, that he had used 
the proportionality of 1° of Fahrenheit to 0-589 inches of the 
barometer, or 530 feet, merely as an approximation for small 
heights. ash 8 
A reference in. Boué’s’ Guide du Geologue Voyageur, di- 
rected me to a paper by Mr Prinsep, in the Journal of the 
Asiatic Society of Bengal for April 1833. I hoped there to 
have found a table of boiling temperatures observed at great 
heights in India. But it only contains a modification of 
Tredgold’s Formula of the Elasticities of Steam adapted to 
the measurement of heights by the thermometer, and no ori- 
ginal observations. 
During a late journey in Switzerland (in 1842), I made 
several observations on the boiling point of water at great 
heights. Having long since abandoned Wollaston’s ther- 
mometrical barometer, as practically useless, I was led to 
resume the method, in consequence of a very ingenious and 
compact apparatus for chemical or culinary purposes having 
been shewn to me the preceding winter, by Mr Stevenson, 
instrument maker, under the name of a Russian furnace, and 
which was, I believe, introduced into the country from Russia 
by Dr Samuel Brown. It consists of a very thin cylindrical 
a ee ee a 
* Phil., Trans. p. 192. + Ibid., vol. ex. p. 295. 
