644 DE. E. EEANKLAND ON THE INELTJENCE OF ATMOSPHEEIC PEESSTJEE 
series, not only that the lines are nearly coincident, but that they do not, down to 14 
inches pressure, deviate much from a straight path. This is obviously due to an equal, 
or nearly equal diminution of light for each equal decrement of pressure down to about 
14 inches, below which pressure both lines deviate markedly from their previous dkection, 
indicating an alteration in the rate of the diminution of luminosity. The mean results 
of the three series of observations give approximately 5T per cent, of the luminosity at 
30 inches pressure as the diminution of light corresponding to each diminution of 1 inch 
of mercurial pressure down to 14 inches. The following Tables exhibit the illuminating 
effect actually observed compared with that calculated from this constant : — 
First Series. 
Pressure in inches 
of mercury. 
Illuminating power. 
Observed. 
Calculated. 
29-9 
100 
100 
24-9 
750 
74-5 
19-9 
52-9 
49-0 
146 
20-2 
220 
9-6 
54 
- 3-5 
6-6 
■9 
— 18-8 
Second Series. 
30*2 
100- 
100 
28-2 
91-4 
89-8 
26-2 
80-6 
79"6 
24-2 
73-0 
69-4 
22-2 
61-4 
59*2 
20*2 
47-8 
49*0 
18-2 
37-4 
38-8 
16*2 
29-4 
28-6 
14-2 
19-8 
18-4 
12-2 
12-5 
8*2 
10-2 
3-6 
—2-0 
Third Series. 
30-2 
100 
100 
29*2 
95-0 
94-9 
28-2 
89-7 
89*8 
27-2 
84*4 
84-7 
The dotted lines in diagrams Nos. 1 and 2 represent the calculated luminosity ac- 
cording to the above Tables. The experimental lines above 14 inches pressm'e do not 
in any part of their course deviate more from the calculated line than might be expected 
from the usual errors of experiment. The law of the diminution of the light of gas- 
flames by reduction of pressure from 30 inches to 14 inches of mercury, may therefore 
be thus stated, 100 imits of light emitted hy a gas-flame burning in air at a pressure 
of 30 inches of mercury^ 5T units are extinguished hy each diminution of one mercurial inch 
