Mr. W. S. Jevons on a Sun-gauge. 355 
1857. February 22. April 26. Sun- 
Hours. f | Black S ale 
mp. lack~ n- Temp. o ack- un- 
— a ‘lb, | See at bulb. gauge. — 
AM. 6°0| 56:2 56°75 0 527 52:0 0 0 
70!) 596 81:7 0 55:1 58:5 0 0 
8:0) 65-2 97-2 0 575 65:0 0 0 
90| 706 106°7 4 62-2 91-4 5 4:5 
10:0} 74:7 111°9 17 65°9 97°5 13 15:0 
11-0 769 116-2 22 70:7 106°8 18 20:0 
12:0} 80-4 120°5 22°5 71-6 104-7 18 20°3 
p.M.10] 79:3 115-2 19°5 72°9 111-5 18 18:7 
2:0| 76:9 107-4 7 73-4 112°8 15 16:0 
3:0] 75:3 93-2 9 71:8 106-0 15 12-0 
40) 74:9 95-9 2 70°4 99:0 8 50 
5:0| 72-4 77-2 0 66-9 67°7 2 10 
60) 70-1 70°9 0 62-1 60°7 05 03 
SS | | 
Means : ahd : : ’ , , 
and sums. haz 96:2 | 113-0 65:6 87:2 1125 112-8 
N.B. The place of observation is shaded till near 8 a.m. by 
elevated ground, hence the absence of indications before that 
hour. The days were neither quite cloudless. 
The results of the sun-gauge given at each hour are the quan- 
tities, in millimetres, of water distilled since the last hour of 
observation. 
The sun-gauge was refilled, with a better vacuum, between 
these two series of observations. 
It must be borne in mind that the present paper is only in- 
tended to communicate the idea of an instrument which it would 
require the help of the most skilful optical instrument-maker to 
carry to perfection, or even to render of utility. 
If the delicacy of the instrument be very much increased by 
enlarging the surfaces of absorption and evaporation and of con- 
densation to a very great extent, compared with the divisions of 
the measuring tube, I see no reason why the instrument should 
not act as a simple and accurate actinometer. 
The amount of distillation during one minute, for instance, 
accurately determined, will give the rate or intensity of the sun’s 
rays, which may be supposed uniform during that period ; just 
as the rate of expansion of a large bulb full of liquid in the acti- 
nometer of Sir J. Herschel is determined by watching and mea- 
suring its expansion in a capillary tube during five seconds of 
exposure to the sun’s rays. 
The total heating effect of the sun’s rays falling on the earth’s 
surface at the different latitudes and localities of the globe, varies 
immensely from several causes, the principal of which are,— 
Ist. The duration of the day, and the elevation of the sun. 
2A2 
