on some of the leading Doctrines of Caloric, Sc. 41 
bulb, equal in length to the column of vapour. It would then 
be necessary that the column of water should rise sufficiently 
above this vapour to allow the thermometer bulb to be equally 
ammersed, or we must make on its itfdications the small correc- 
tion mentioned p. 59, in order to reduce the temperature of the 
cylinder of mercury to the temperature of the reservoir. The 
employment of such a thermometer may appear at first sight 
sufficiently difficult, since it seems that the length of the cylin- 
drical reservoir must be very considerable, if the elastic force of 
the vapour be great*.” 
He then proceeds to show how this difficulty may be obviated 
(as indeed it had previously been by Mr. Dalton), by taking 
barometer tubes successively shortened, as the force of the steam 
is augmented by heat. He proposes to use four, between the 
freezing and boiling points of water, each being two decimeters, 
or nearly 8 inches long, and the thermometer bulb having also 
that length. The plan which I imagined, as it completely ob- 
viates the source of errors arising from the large and variable 
space occupied by the vapour, supersedes the necessity of employ- 
ing M. Biot’s singular remedy. It likewise avoids other com- 
plications, introduced by the heating and consequent elongation 
of the mercurial column itself attending all the other methods ; 
and scarcely capable of being exactly appreciated at high tempe- 
ratures with the apparatus of Professor Robison. 
The space over which the vapour extends in my instrument, 
need never be greater than half an inch of the barometer tube, 
against the side of which part the oblong bulb of a delicate ther- 
mometer rests, so as to indicate the true temperature. And 
though the liquid and incumbent vapour are thus always restricted 
to the summit of the barometer tube, we can, notwithstanding, 
measure its progressive range of elasticity, from zero of Fahren- 
heit to one hundred, or even two hundred, degrees above the 
boiling point of water, from an elasticity of 0.07 of an inch, to 
that capable of sustaining 14 feet, or even 36 of mercury. Fig.1 
(Pl. J.) represents the construction employed for tempe- 
ratures under and alittle above the boiling point. Fig. 2 and 
3 are used for higher temperatures; the last is the more con- 
venient of the two. Each was suspended from a lofty window 
ceiling, and placed in atruly vertical position by means of a 
plumb line. 
One simple principle pervades the whole train of experiments ; 
which is, that the progressive increase of elastic force developed 
by heat from the liquid, incumbeut on the mercury at J 1’ 1", is 
measured by the length of column which must be added over L, 
* Traité de Physique, tome i. p. 268. 
+ This Plate will be given with our next Number. 
tue 
