1819.] ~ Scientific Intelligence. 469 
the repetition of experiment than to cast the apparatus into 
shade. 1 repeated the experiment of the ebullition of water on! 
the summits of the Simplon and Mount Cenis, but the results 
were by no means approximative. The barometric measurement 
of the former may be questioned, but the latter I cannot hesitate 
to admit, confident of the accuracy of the observer (from a per- 
sonal acquaintance with him), Dr. Frederick Schow.* ‘The 
comparisons were made with the cotemporaneous observations of 
the Baron de Zack, at Genoa. As a question might, however, 
arise on the thermometers I employed, from their being without 
a minute graduation or a nonius, | shall beg simply to give you 
the result of the experiment made at the village of the Simplon 
on the evening of the 15th August last, with Capt. B. Hall. The 
bulb of the thermometer maintained in contact with the vapour 
of water in ebullition indicated a temperature of 201°6°, corre- 
sponding with 24:45 inches of the barometer, and equal to 
5,400 feet altitude, being an excess of 577-75 feet above the 
barometrical height observed. Moreover, there should be a 
compensation for the expansion of the thermometric bulb; and if 
it be true, agreeably to the experiments of M. Gay-Lussac + 
(though these have been questioned in some recent researches), 
that water boils at a lower temperature in metallic than in glass 
vessels, some note should be taken of this, and these circum- 
stances would increase the excess. 1 think a metallic scale from 
its expansion, unfavourable to accuracy of result. 
I can conceive two causes operating against the results we 
would willingly accept. One of these is an interim change of 
density in the atmosphere. It would then require the compen- 
sation of the barometer, an instrument it was intended to 
supplant. A series of experiments should, therefore, be made to 
indicate on the level of the sea the points of ebullition for the 
barometrical range, and the degree of ebullition on the given 
height again compared with contemporaneous observations on 
the level of the sea, or other well ascertained position as noted by” 
the barometer. This would not, however, account entirely for 
so enormous a difference which, on the great St. Bernard, 
amounted to about 1000 feet. The hygrometric state of the 
medium strikes me as the chief cause. Dew forms only in the 
valley, not on the mountain top; and experiment has amply 
confirmed the extreme siccity of the atmosphere at great eleva- 
tions. By providing the rarified medium with an absorbin 
material, Professor Leslie has ingeniously and beautifully effecte 
the congelation of water. By a parity of reasoning, we infer 
* Dr. Schow is one of the scientific gentlemen employed by order of the King 
of Denmark. ‘His department is the determination of the geographical position 
of plants, so ably begun by Baron de Humboldt. To his unwearied and uninter- 
mitting exertions I cheerfully pay my tribute of admiration, 
+ Any opposition to the experiments of this very acute observer must be 
zeceived with all due caution. 
