Ee Sree eee 
a any) ME 
Oe ee ee a ee ee 
NE ae ee Se ae a 
ee a ee aR Tee ET ae ee ee 
Colombia between the years 1820 and 1830. 23 
Working upon the foregoing principle, Caldas adapted to his 
thermometer a barometrical scale. The product of 0°.974 of 
Reaumur by 19 is 18.506, or, in round numbers 18.5, i. e. 18°.5 
of Reaumur corresponds to 19 inches of the barometer. Then 
measuring 18.5 from the summit, or 80° of Reaumur’s scale, he 
transferred it to the opposite side of the thermometer, dividing it 
into 19 equal parts, or inches of the barometer, subdividing 
these by a nonius into 24 each = half a line of the barometer. 
In this manner the elevation of the thermometer by boiling 
water indicates the corresponding elevation of the barometer 
under the same atmospheric pressure. Caldas observes that 
Humboldt, to whom he had communicated these ideas, when 
they met in Popayan, objected the variability of the heat of 
boiling water under the same atmospherical pressure ; to which 
he replies: “ Long practice has taught me its invariability in 
this respect, using the requisite precautions in making the ex- 
periment: otherwise, how could there be equal thermometers? 
Is not the invariability of the heat of boiling water under the 
pressure of twenty-eight inches, the foundation of the superior 
term of all thermometrical scales? It is true that boiling water 
does not immediately acquire its extreme heat, but pushing the 
Operation to its maximum its heat is always the same.” p. 24 
Caldas did not consider an invariable exponent possible, on ac- 
count of the variability of atmospheric pressure. ‘The want, 
however, of a barometer induced me to make some experiments 
to this effect, by way of rendering this method of measuring el- 
€vations still more simple, and of more general use. Is the va- 
niability of atmospheric pressure such as to make any important 
difference in these calculations? Does not water boil constantly 
at 212° at the level of the sea? At Quito I found the same re- 
sult as Caldas had several years before; and several times the 
‘Same result in this and other parts of the Andes. The difference 
then, is scarcely perceptible in the thermometer, and consequently 
unimportant in the results of a calculation founded on the heat of 
boiling water. The thermometer besides, immersed in boiling 
water, is less liable to a variety of atmospheric influences to 
which the mercury of the barometer is necessarily subject. 
Hence the great differences in different barometrical measure- 
ments of the same elevations, and the differences observed be- 
twixt different thermometers exposed to the air in the same place, 
