352 Mr. W. S. Jevons on a Sun-gauge. 
sive, and difficult to use; and the common black-bulb thermo- 
meter, which is generally employed, does not give results of any 
direct value or comparability. 
But it appears to me, that even if we did possess a convenient 
true actinometer to give the intensity of the sun’s rays at any 
given place and moment, that is, the rate of the sun’s heating 
power, there is still a second instrument required in meteorology 
to measure the accumulated effect of the sun’s rays during any 
given space of time. In short, the sun’s heat should be gauged 
at every meteorological observatory as it arrives, day after day 
and year after year, precisely in the manner that falling rain 1s 
collected by the rain-gauge, and its accumulated amount mea- 
sured at the end of any given period. 
This instrument, which I propose to call a sun-gauge, is 
merely an adaptation of the invention familiarly known as Wol- 
laston’s Cryophorus. If the sun’s rays be allowed to fall upon 
the surface of water contained in one of the bulbs of this instru- 
ment, the other bulb being sheltered from the sun but freely 
exposed to the air, the amount of water evaporated from the 
former and condensed in the latter bulb will afford a simple and 
perfect measure of the total amount of heat absorbed by the 
surface of water. For when pure water is contained m any 
exhausted vessel, the tension of the vapour therein can never 
remain above that due to the temperature of the coldest part of 
the vessel, otherwise rapid evaporation and condensation are 
produced until equilibrium is attained. When a moist surface 
therefore is exposed to the sun in a vacuous glass vessel, of 
which one part is sheltered from his rays and placed in a free 
current of air, the tension of the vapour within the glass can 
never rise perceptibly above that corresponding to the tempera- 
ture of the air, and all the sun’s heat absorbed by the exposed 
part will be carried over in a latent state into the sheltered part 
by the evaporation of a definite amount of water. The mere 
addition of a graduated measuring tube to Wollaston’s cryo- 
phorus will thus convert it into a heat-gauge. 
The only instrument which I have been able to make (without 
the aid to be obtained in London or other towns) is of a simple 
and rough form. It consists of an oblong glass-bulb, about 3 
inches in length and 12} inch in diameter, into the mouth of 
which a half-inch glass tube is fused, in such a manner that a 
part of the tube reaches within the bulb 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 millimetres, which read from the further end. 
The water, which may be rendered opake by some fixed colour- 
ing matter, such as a weak solution of sulphate of indigo, is to 
