Mr. W. S. Jevons on a Sun-gauge. 353 
be introduced in sufficient quantity to fill about two-thirds of the 
bulb, and the instrument being then perfectly exhausted of air 
by boiling, is completed by hermetically 
sealing the extremity of the graduated glass 
tube. 
To use the sun-gauge, the whole of the 
liquid must first be poured into the bulb, 
which can readily be done on account of the 
bent position of the interior part of the tube. 
It is then inverted and placed in some sup- 
port where the sun’s rays may fall upon the 
bulb uninterruptedly throughout the day, 
while the tube beneath is completely shel- 
tered from the sun, but freely exposed to 
the air. This may easily be done as shown 
in the figure, where the graduated tube 
hangs down into a box open only towards 
the side on which the sun never shines. 
It is evident, that as long as the sun 
shines, or the heat of his rays is at all per- 
ceptible, water will evaporate from the bulb 
and condense colourless and pure in the , 
graduated tube, by the divisions of which \ 
its amount may be quickly and easily read 
off. Except so far as an imperfection of the instrument, to be pre- 
sently mentioned, may interfere, the results thus given by the 
same instrument will both be exactly proportional to the amounts 
of heat absorbed, and will also enable us to determine and ex- 
press the absolute amount in a constant and convenient manner. 
But in constructing a number of instruments, the proportion of 
the sun’s rays absorbed and gauged by the exposed bulb may not 
always be the same; so that it will be necessary to compare them 
against some standard instrument, and thus determine for each 
a correction factor which will reduce all results to complete 
uniformity. 
Amounts of radiated solar heat may be conveniently expressed 
by the depth (in inches or millimetres) of evaporation from a 
surface of water exposed with perfect freedom to his rays, under 
the condition that all aqueous vapour of a greater tension than 
‘199 inch of mercury (the tension of the temperature of 32°) 
shall be immediately conveyed away. Or the relation of this wnit 
inch of water to the actine*, or “ the abstract unit of solar radia- 
* The actine is “that intensity of solar radiation which, at a vertical 
incidence, and supposing it wholly absorbed, would suffice to melt one- 
millionth part of a metre in thickness from the surface of a sheet of ice 
horizontally exposed to its action per minute of mean solar time.” —Admi- 
ralty Manual, p. 307. 
Phil, Mag. 8. 4. Vol. 14. No. 94. Nov. 1857. 2A 
