On the Climate of Strathpeffer Spa. 49 
may suitably introduce this Jast portion of the present 
essay. 
The latitude of a given place, though only one of many 
factors that combine to form what we term its climate, must 
exercise an influence of no small importance, for upon it 
depend the length of the day and the altitude of the sun. 
Differences of latitude are therefore the immediate cause of 
corresponding differences in both the duration and intensity 
of sunshine. By sunshine we understand solar radiation, in 
all its properties, thermal and chemical as well as illuminat- 
ing. It would be of no small convenience in climatology if 
we possessed some ready means of stating and comparing the 
amount of this radiation at different latitudes, embracing as 
it must do the two elements of intensity and time. Many 
years ago it occurred to my father, Dr J. J. Fox of Hertford, 
that the successive momentary portions of sunshine given 
forth during the day at continually-varying distances from the 
zenith could be integrated and expressed in terms of the 
number of seconds that the sun would require to shine zn the 
zenith to impart to the earth an equal amount of radiant 
energy. The advantage of reducing the expression to zenith 
solar seconds is that it furnishes a perfect standard of com- 
parison, which is applicable to all declinations of the sun and 
to every latitude. My father has kindly made calculations 
for the latitudes of Strathpeffer and Greenwich, some results 
of which are exhibited in Table VIII., and graphically in the 
TABLE VITI. 
Zenith Solar Seconds. 
Length of 
ay. 
Hours and 
| In Seconds. tases | gmat reais tig | | 
e / 
Winter Solstice— | H. M. | H, M. 
Strathpeffer, ad 2340 0 39 . ae ) 
Greenwich, . , ail 4620 | 1. i: 36 ) 
Equinox— | | 
Strath peffer, : ay | 14760 4 6 ma 66 | 
Greenwich, . : ; 17100 4 45 12-0 
Summer Solstice— | 
Strathpeffer, iced 31320 8 42 17 43 
Greenwich, . a ; 31560 8 46 16 24 
VOL. XI. | D 
