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PHIPPS’s JOURNAL, 571
come a true thermometer, in which the interval between the points, anfwering to melting
ice and boiling water, would be divided into forty parts. If, therefore, a thermometer,
with a fcale fimilarly divided into forty parts between the fixed points, be placed near
the hygrometer, it will {hew immediately the corre€tion to be made on that inftrument
for its variation as a thermometer; with fome reftrictions however; of which M. De
Luc has given an account in the paper he fent to the Royal Society on the fubje& of
this hygrometer.
‘That part of the frame of the inftrument on which the feale is marked, is moveable ;
fo that, before obferving the points at which the mercury ftands, it may be pufhed up-
wards or downwards, according as the thermometer has rifen or fallen with refpeé to
the point of melting ice: and thus the indications of the hygrometer can at once be
freed from the errors which would arife from the difference in the volume of the quick-
filver, on account of the different degrees of heat.
Defcription of the Manometer, confiruétcd by Mr. Ram/fden.
Tue manometer ufed in this voyare was compofed of a tube of a fmall bore, with a
ball at the end; the barometer being at 29,7, a {mall quantity of quickfilver was pnt
into the tube to take off the comminication between the external air, and that confined
in the ball and the part of the tube below this. quickfilver. A {cale is placed on the
fide of the tube, which marks the degrees of dilatation arifing from the increafe of heat
in this {tate of the weight of the ait, and has the fame graduation as that of Fahren-
heit’s thermometer, the point of freezing being marked 32. In this {tate therefore it
will fhew the degrees of heat in the fame manner as a thermometer. But if the air
becomes lighter, the bubble inclofed in the ball, being lefs compreffed, will dilate itfelf,
and take up a fpace as much targer, as the compreiling force is lefs; therefore the
changes arifing from the increafe of heat will be proportionably larger ; and the in-
ftrument will fhew the differences in the denfity of the air, arifing from the changes in
its weight and heat. Mr. Ramfden found, that a heat, equal to that of boiling water,
increafed the magnitude of the air from what it was at the freezing point .° of the
whole. From this it follows, that the ball and the part of the tube below the
beginning of the {cale is of a magnitude equal to almoft 414 degrees of the fcale.
If we have the height of both the manometer and thermometer, the height of the
barometer may be thence deduced by this rule; as the height of the manometer in-
creafed by 4) 4, is tothe height of the thermometer increafed by 4143 fo is 29,7, to
the height of the barometer.
‘This inftrument, though far from complete, having been conftructed in a hurry for
the purpofe of a firlt experiment, and liable to fome inaccuracies in the obfervations
from not having the thermometer with which it was compared attached to it, feldom
differed from the marine barometer ,!; of an inch. Should it be improved to that
degree of accuracy of which it feems capable, it will be of great ufe in determining
refractions for aftronomical obfervations, ‘as well as indicating an approaching gale of
wind at fea.
402 MISCEL»
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