MAGAZINE OF SCIENCE AND ART. 
59 
•weight of watery vapour, or which is the same thing, of 
water, conveyed from ono bulb to the other (at 32 de¬ 
grees), we have only to multiply it by 1092, to express 
as simply as it can be expressed, the amount of heat 
carried over. The mere addition of a graduated glass 
measuring tube to one of the bulbs of the cryophorus, 
thus converts it into an effectual heal~gaugc, 
To apply it as a sun-gauge, it is only necessary to 
allow the sun’s rays to fall upon the surface of the water 
contained in one of the bulbs, while the other is shel¬ 
tered from the sun, but freely exposed to the air. In 
this case, the tension of vapour within the instrument 
ran never rise perceptibly above that corresponding to 
the temperature of the coldest part, viz,, that cooled^ by 
the air. Thus all heat absorbed from the sun’s rays 
by the exposed bulb will be immediately converted into 
latent heat, and conveyed across into the other bulb, by 
distillation, and as I have just explained, the actual 
quantity of heat thus _ conveyed over, may be directly 
determined by measuring the amount of water caused to 
evaporate. 
The principle of this sun-gauge I may say, is theo¬ 
retically perfect, for as no part of the instrument should 
rise in temperature perceptibly above that of the air, 
F^. 
J 
fig 2 - Fig 3, 
710 interference will he produced by the conduction of 
heat. But to carry the principle into practice success¬ 
fully, the following qualifications are necessary in the 
instrument, viz.:— 
First, a surface of an invariable and determined area 
and nature to absorb the direct rays of the sun, but not 
exposed to radiation from any other direction. If the 
sun’s heat-rays be not entirely absorbed, it is sufficient 
if a constant proportion of them be so. 
Secondly, a surface of water, likewise of constant and 
determinate area, to which the heat so absorbed shall bo 
instantly communicated, in order that it may he con¬ 
verted into the latent state, and carried off by evapora¬ 
tion. It will no doubt be most convenient and simple 
to make the surface of water itself the absorbent area 
(as in the instruments to be presently described.) 
Thirdly, a condensing surface of considerable extent, 
which shall be continually maintained at the tempera¬ 
ture of the air. 
Fourthly, a graduated tube to measure with sufficient 
accuracy the quantity of water collected in the conden¬ 
ser, and thus to determine in an almost direct manner 
the quantity of heat received by the absorbent surface. 
The only instrument which I have been able to con¬ 
struct is a mere simple and rough model, in which all 
these four parts are confused together. It consists, that 
is figure IT. of the diagram, fsee Jig. I .) of an oblong 
glass bulb, about 3 inches in length‘and 1^ in diameter, 
into the mouth of which, a half-inch glass tube has 
been fixed in such a manner that a part of the tube 
reaches within the bnlb almost to its opposite end, being 
bent, however, towards one side. The exterior part of 
the tube, 14 inches (or more) long, is straight, and 
graduated into milemeters, which read from the bottom. 
