562 Proceedings of the Royal Society 
ence of temperature between the warmest and coldest months of 
the year. 
For a study of diurnal range we should require a series of ob- 
servations for every hour of the day and night from all parts of the 
earth, and the places where such laborious observations have been 
made are as yet very few; hut the returns of daily and monthly 
temperatures now obtainable from all countries are sufficient for a 
tolerably complete study of the simpler head of annual range. 
It is believed that the subject of annual range has never before 
been systematically worked out for any large portion of the earth’s 
surface, though its general conditions may have been recognised 
from the comparison of a few isolated points. 
In preparing the'charts of annual range now shown, it was first 
assumed that the months of January and July are respectively the 
coldest and warmest months of the northern, and the warmest and 
coldest months of the southern hemisphere. This proves to be the 
case, with few exceptions, in all parts of the earth, excepting those 
near the equator, which have two maxima and minima; but the 
annual range of temperature in these regions is so very small, that 
the difference between the temperatures of January and July may 
be taken as the measure for every part of the earth where range is 
considerable. January and July temperatures were then collected 
from all available sources, and their differences being taken, the 
annual range of twelve hundred places in all countries was ob- 
tained. These figures were then set down on maps, on the positions 
of the places of observation, and lines were drawn through those 
which have an equal range, at intervals of ten degrees of increase,* 
thus presenting the subject in a graphic form. The places of' 
* 
observation in Europe are so numerous as to enable the lines to be 
laid down there with almost absolute precision. Then the collection 
of temperatures throughout the United States, Mexico, and the 
West Indies by Laurin Blodget, the returns from Canada and the 
Forts of the Hudson Bay Company, observations for a long series 
of years at the Danish stations in Western Greenland, and the 
* In the reduced chart which accompanies this paper the lines of 10, 30, 
50, 70, and 90 degrees of range have, for the sake of clearness, been omitted, 
but the course of these intermediate lines may perhaps be traced from the 
description which follows. 
