914 PEOFESSOE BIJNSE]?? AKD DE. H. E. EOSCOE’S PHOTO-C:HE:^^CAL EESEA.ECHES. 
Table XIX. 
O’. 
10’. 
20’. 
.30’. 
40’. 
50’. 
60’. 
70’. 
80’. 
90’. 
m 
0-80 
132-5 
130-7 
125-2 
115-7 
102-3 
81-4 
55-2 
24-5 
2-1 
i 
0-0 
0-75 
139-8 
138-2 
132-7 
123-2 
108-9 
88-1 
6l-5 
28-8 
2-8 
0-0 
0-70 
147*8 
146-1 
140-7 
131-3 
116-9 
96-5 
68-7 
33-8 
3-8 
0-0 
0*65 
156-2 
154-5 
146-6 
139-6 
125-7 
105-2 
76-6 
29-2 
5-3 
0-0 
0-60 
165-0 
163-3 
158-1 
149-0 
135-0 
114-5 
85-5 
46-6 
7-2 
0-0 1 
0*55 
174-2 
172-6 
167-6 
158-7 
145-0 
124-7 
95-4 
54-7 
9-9 
0-0 , 
0-50 
184-1 
182-5 
177-7 
169-0 
155-7 
135-7 
106-4 
64-2 
13-6 
0-0 i 
0-45 
194-4 
193-0 
188-4 
180-1 
167-2 
147-8 
118-8 
75-3 
18-6 
0-0 
0-40 
205-3 
204-0 
199-6 
191*9 
179-7 
161-0 
132-5 
88-4 
25-5 
0-0 
0-35 
216-9 
215-7 
211-6 
204-4 
193-0 
175-3 
147-8 
103-8 
35-0 
0-0 ; 
0-30 
229-1 
228-0 
224-4 
217-8 
207-2 
190-9 
165-0 
121-7 
48-0 
0-0 
0-25 
241-9 
241-0 
237-8 
231-9 
222-6 
207-9 
184-1 
142-9 
65-8 
0-0 1 
0-20 
255-7 
254-8 
252-1 
247-2 
239-1 
226-3 
205-3 
167-7 
90-1 
0-0 j 
0-15 
269-5 
269-4 
267-3 
263-2 
256-8 
246-5 
229-1 
196-9 
123-5 
0-0 1 
0-10 
286-2 
284-7 
283-2 
280-5 
275-8 
268-4 
255-7 
231-1 
169-3 
0-0 ! 
0-05 
301-3 
301-0 
300-2 
298-7 
296-3 
292-2 
285-2 
271-1 
232-1 
0-0 ; 
0-00 
318-3 
318-3 
318-3 
318-3 
318-3 
318-3 
318-3 
318-3 
318-3 
0-0 
From the curves we also see that the variations in the illumination appear most 
strikingly the lower the sun is above the horizon. When, for instance, at Eeykiavik, 
under a barometric pressure of 0‘"'770, the sun is 10° above the horizon, the ground is 
illuminated by direct sunlight to 2 ’5 degrees of light ; if the barometer sinks 10 millims., 
the illumination rises to 2 '9 degrees of light. On the highest point of the neighbouring 
crater of Hecla, upon which one of us observed a barometric pressure of 0™'6290 on the 
26th of July, 1846, the illumination from direct sunlight must rise to 6T, and'on the 
summit of Dwalaghiri to at least 90 degrees of light. At the time when the sun has 
nearly reached the zenith in the latitude of the Himalayas, the amount of the direct 
sunlight which falls on the valleys of the Thibetian high-lands where grain is cultivated, 
is nearly one and a half time as large as that falling on the neighbouring low-lands of 
Hindostan. This difference increases in so rapid a ratio mth increasing zenith-distance, 
that when the sun is 45° removed from the zenith, the direct solar rays on the high table- 
land of Thibet give more than twice the chemical action of those falling on the plains 
of India. 
From these few examples it is seen how rapidly the chemical intensity of the sunshine 
increases with considerable elevation above the sea’s level. Yet the variations effected 
by reason of these changes are small when compared with the differences brought about 
by alterations in the latitude of the place of observation. In order to give an idea of 
these variations, we have calculated the chemical action which the sun’s rays effect 
under a barometric pressure of 0™’76 at the spring equinox during every hour of the day, 
on a horizontal unit of area situated at the points on the earth’s suiJace for which the 
chemical illumination of the whole diffuse light has already been determined. The 
results of this calculation are contained in Table XX. The formula 
0-4758 P 
