TOPOGRAPHY. XV 
56°30’, and Orkney in 59°, a difference of 2}°. Whilst 
some plants show a wider range than others, we 
are within the mark when we take it that there 
is a difference of 600 feet in range above sea-level 
between the same plants in Perth and Orkney, or 
240 feet for every degree. 
As our winter temperature is higher than that 
of Perth, and our sunshine little less, the difference 
in range of plants is solely due to the difference 
in summer temperature. The following statement 
shows this difference in temperature for four 
months :— 
Perth. Deerness, Orkney. 
January .......... Si Ot fc shes wens 39°0 
Weta eeeerey eemeee Be a Bete eck: 42°4 
San meee eee Soli 2 enuloa 54°2 
September....... Basay |p a seeees 51°5 
The comparison of temperatures shows that 
Orkney is nearly 2° milder in winter and 43° 
colder in midsummer than Perth. This summer 
temperature is the factor which determines the 
creat difference in the altitude at which the same 
plant is found in different localities. The mean 
annual rainfall for Orkney is 36°65 inches, and for 
the city of Perth, 32:04; but for Ochtertyre, Perth- 
shire, it is 41°42. The mean annual hours of sunshine 
for Orkney are 1185, and for Aberdeen and Fort 
Aucustus—the nearest available sunshine stations to 
Perth—respectively 1401 and 907. 
The statement below gives the monthly and 
annual means for temperature, sunshine, and rain- 
fall for Orkney :— 
