Actinometrical observations from Greenland. 3761 
or sometimes at the beginning of May the sea-ice breaks up and 
the sky becomes cloudy, but the snow-covered ground as yet reflects 
much light. Not until June do the sea-ice and the snow-covering of 
the country disappear, and the spring climate is often very unsettled, 
a fact which often reduces the light-totals for June and July. From 
the end of July to the beginning of September the sky is usually 
clearer but, at the same time, the altitude of the sun's path is very 
rapidly decreasing. The end of autumn and the beginning of winter 
is again very unsettled with frequent cloudy days, but now the sun 
is so low that the cloud-screen as we have seen, plays almost no 
part at all with regard to shutting out the chemical light-rays. 
The effect of the different circumstances pertaining to cloud- 
screens in both the places we are here considering is especially 
instructive around the equinoctial periods. Day and night are equal, 
but the altitude of the sun at midday on Anholt is 12°-5 greater 
than on Disco. During autumn the daily light-intensities 
on Anholt are about four times as great as those on 
Disco, while during spring they are twice as low. The 
reason for this is evidently that the spring equinox on Disco is 
favoured by settled, clear weather while the weather at the autumn 
equinox is usually unsettled. 
The influence of the long Polar-day. 
If the hours during which the sun is above the horizon be 
termed “day” then the annual total sum of “day’-hours should be 
equal to the sum of “night”-hours, and these sums should be equally 
large at all places on the earth’s surface, But by refraction the 
sum of “day”-hours is somewhat increased and almost proportion- 
ally to the increase in latitude. From this it may be expected 
that the annual light-totals for the Arctic regions must be higher 
than those for the Equatorial regions; but as the light, and more 
especially the actinically-active rays, depends so much upon the 
altitude of the sun the reverse is really the case — the twelve-hours- 
long tropical day, with altitudes of the sun up to 90°, will probably 
show a still higher chemical light-total than the 24-hours-long Polar- 
day. On the other hand the length of a Polar-day is a considerable 
compensation for the low altitudes of the sun. Instances of this 
may be easily found in the Tables and in the accompanying table of 
curves. 
The description of the Graphic table (р. 372). 
The two upper curves show the altitude of the sun in the 
meridian above Anholt and South Disco, the next two the length- 
