354 Mr. W. S. Jevons on a Sun-gauge. 
tion,” adopted by Sir J. Herschel, having been once determined, 
all the results could of course be equally well expressed accord- 
ing to this unit, leaving out the consideration of ¢ime. In prac- 
tice, the vapour in the sun-gauge will not be removed by con- 
densation in the sheltered part of the apparatus till its tension 
rises above that of vapour at the temperature of the air. As, 
however, the latent heat of vapour diminishes as the sensible 
heat rises, a small correction must be made according to the 
mean temperature of the air during the time of observation, as 
given by the following Table :— 
Temperature of air. Specific heat of Correction factor. 
vapour. 
32 1092 000 
42 1085 006 
52 1078 013 
62 1071 020 
72 1064 026 
82 1057 032 
92 1050 ‘038 
102 10438 045 
If a = observed result of instrument, 
True result =a—a x correction factor. 
One defect of the particular form of sun-gauge here described 
is, that when evaporation has proceeded to a considerable extent, 
the quantity of liquid in the bulb is diminished, and the absorb- 
ent area lessened in comparison with the surface of evaporation, 
so that the distillation of a millimetre’s depth of water at the 
commencement of the exposure will indicate less amount of heat 
rays than the same distillation towards the end of the day. A 
definite and invariable horizontal surface of water is all that 
should be exposed to the sun’s rays, and by an instrument of 
more complicated and perfect form this might perhaps be accom- 
plished. If not, the inaccuracy might be experimentally deter- 
mined for each instrument, and a tabular correction applied for 
each division of the graduated tube. 
Likewise, in order to obtain very accurate and constant results, 
great care will be necessary with this instrument, as with the 
black-bulb radiating thermometer, that it may not be affected 
by heat communicated from the support or other neighbouring 
objects. This has not been much attended to with the present 
rough form of the sun-gauge. 
The following are results obtained with the sun-gauge during 
two days’ continuous observations :— 
