616 
PEOFESSOE EOSCOE ON A METHOD OF METEOEOLOGICAL 
band is introduced into the bag in the form of a coil, then unwound and pushed into 
the slide. 
The reading-instrument is represented by fig. 6. It consists essentially of a metallic drum 
80 millims. in diameter and 37 millims. broad, upon which a piece of thick white cartridge 
paper, and over it the graduated strip, is fastened. The edge of the drum is furnished 
with a millimetre-scale, and the dark end of the strip is made to coincide with the com- 
mencement of the scale. The drum turns upon a horizontal fixed axis against a vertical 
circular plate (N), being held in position by the screw (O). The drum and vertical plate 
are fixed upon a pillar and foot (P). The insolation-band is held against the graduated 
strip by means of two spring clamps (QQ'), placed apart at a distance of 130 millims. and 
fixed to the vertical plate (N). By moving the drum on its horizontal axis, the various 
shades of the fixed strip can be made to pass and repass each of the holes on the insola- 
tion-band, and the points of coincidence in tint on the strip and each of the insolated 
papers can be easily ascertained by reading off by the light of a soda-flame in a dark 
room. The lens (B.) fixed upon the brass pillar of the instrument serves to concentrate 
the light from the flame upon the small surface under examination. If a coal-gas flame 
can be procured at the Observatory, the best mode of obtaining the monochromatic light 
is to place two beads of sodic carbonate upon fine platinum loops into the colourless 
flame of a Bunsen burner ; if a coal-gas flame cannot be obtained, the flame of a lamp 
fed with spirit saturated with common salt can be used, and beads of the more volatile 
sodic chloride held into the flame. The reading of each observation is made ten times, 
and the mean of these readings taken as the result. 
The following observations of the intensity of the chemical action of light on July 8, 
1864, may serve as an example of the detail of the determinations. 
Solar time. 
T. 
Duration of 
exposure, 
Mean 
reading, 
R. 
Tabulated 
intensity 
of strip, 
Calculated 
intensity, 
n 
Condition of 
solar disk. 
Amount of 
cloud. 
Barom. 
Temperature. 
Dry 
bulb. 
Wet 
bulb. 
h m 
millims. 
7 10 A.M. 
18 
96 
1*00 
0-055 
Clouded over 
8 
7 50 
15 
93 
1*03 . 
0-068 
Clouds 
7 
8 25 
12 
90 
1-06 
0-089 
j> 
9 
9 0 
10 
76 
1-20 
0-12 
jj 
„ 
9 30 
10 
75 
1-21 
0-12 
„ 
millims. 
10 30 
10 
64 
1*33 
0-13 
„ 
765-1 
18-6 C. 
13-9 C. 
11 0 
10 
76 
1*20 
0-12 
Clouded over 
10 
11 30 
10 
67 
1-30 
0-13 
„ 
„ 
12 0 
10 
86 
M0 
0-11 
„ 
18-7 
13-3 
12 30 p.m. 
6 
107 
0-78 
0-13 
Light clouds 
9 
19-3 
13-5 
1 10 
8 
73 
1-24 
0-15 
„ 
7 
1 40 
5 
105 
0-80 
0-16 
„ 
19-3 
13*7 
2 15 
4 
93 
1-03 
0-26 
Unclouded ... 
4 
19-7 
13-9 
3 0 
4 
80 
1-16 
0-29 
» 
3 
20-0 
14-4 
3 30 
21 (with disk) 
99 
0-93 
0-26 
4 0 
5 
86 
M0 
0-22 
n 
21-1 
14-4 
4 30 
8 
76 
1*20 
0-15 
1 
5 0 
11 
66 
1*31 
0-12 
6 10 
60 
116 
0-66 
0-011 
» 
” 
