On the Climate of Strathpeffer Spa. 51 
to that which would be given forth if it were stationary in 
the zenith for 1 hr. 17 mins. Again, at Strathpeffer Spa, 
the shortest day has a length of 6 hrs. 17 mins., and the 
zenith solar seconds, representing the aggregate power of the 
sun for that day, amount to just 39 mins. At the equinoxes, 
whilst of course the length of the day is the same at the two 
stations, the zenith value of the solar radiation is 4 hrs. 
45 mins. at Greenwich against 4 hrs. 6 mins. at Strathpeffer. 
This reduction at the more northern station is due simply to 
the lower altitude of the sun, and the consequently greater 
obliquity of its rays. Lastly, at the summer solstice the 
length of the day at Strathpeffer has expanded to 17 hrs. 
43 mins., being 1 hr. 19 mins. longer than at Greenwich ; 
whilst the zenith solar seconds are nearly equal, amounting 
to 8 hrs. 42 mins., against 8 hrs. 46 mins. at Greenwich. It 
follows that in summer the greater length of the day in the 
north of Scotland nearly compensates the loss of radiation 
involved in the lower altitude of the sun; whilst in mid- 
winter, on the contrary, the lower altitude and shorter day 
combine to reduce the radiation to nearly one-half the amount 
at London. The diagram is intended to exhibit the same 
facts by aid of curves. The full-curved lines represent 
respectively the length of day and the zenith solar seconds 
(zenith values) for London, and the dotted curves those for’ 
Strathpeffer. 
If the surface of the earth were smooth and homogeneous, 
and there were neither sea nor air upon it, it would be a 
comparatively simple matter to assign the climate of each 
parallel of latitude by ascertaining the zenith solar seconds 
proper to that parallel. We have, however, already seen how 
powerfully both temperature and rainfall are affected by 
causes independent of latitude, by proximity of sea or of 
mountains, by oceanic currents and currents in the atmosphere, 
or winds. These influences may be so strong as to compen- 
sate the effects of diminished solar radiation, and give toa 
northerly station, such as Stornoway, the same degree of 
warmth as is enjoyed by places many hundreds of miles 
farther south. 
There is, however, another powerful factor of climate which 
