38 
HINTS TO TRAVELLERS. 
that of an average day in a long series of Januaries. Hence it is not 
likely that the exact distribution of temperature shown in the map will 
ever be found on any January day; but it is to be expected that most 
days in every January will have a distribution of temperature which is 
very similar to that shown. The same is of course true of maps showing 
pressure, or rainfall, or any other average condition. 
Again, the isotherm is necessarily constructed from average tempera¬ 
tures which have been corrected so as to be applicable to the same level. 
On the equator, for instance, the summit of a lofty mountain is seen by 
the snow on it to have a temperature not exceeding 32° E., while at sea- 
level the temperature may be 90°. But observations have been made 
showing the rate at which the temperature of the air diminishes as the 
height increases, and although the rate varies in different places and at 
different seasons, it may be taken as about one Fahrenheit degree in 
300 feet. Now if the mountain top with a temperature of say 30° F. is 
known to be 18,000 feet above the sea, the addition of 1 ° for every 
300 feet, or 60° altogether, would give the temperature of 90° as that 
corresponding to sea-level. By applying such corrections, the isothermal 
maps have been constructed to show the distribution of temperature at 
the level of the sea. In order to compare the temperature he has observed 
with that on the map the observer must calculate the average of his 
daily observations for the month in question, and then make the correc¬ 
tion for the altitude of his station. 
Similarly, in ascertaining from an isothermal map the mean temperature 
of a particular place, care must be taken to subtract from the number of 
degrees of the isotherm passing through the place one degree for every 
300 feet of elevation. Of course it will usually happen that no isotherm 
as shown on the map runs through the point the mean temperature of 
which it is desired to obtain. In that case the temperature at the point 
will be found by considering its relative position between the two nearest 
isotherms. Thus, if it lie half-way between the lines of 60° and 70°— 
measured perpendicularly to the isotherms—the temperature of 65° may 
be assumed; if it lies one-tenth of the distance from 60° and nine-tenths 
from 70°, it is safe to assume 61°; if three-tenths from 70° and seven- 
tenths from 60°, then assume 67°. If the point lie in a loop of a single 
isotherm, e.g ., Cape St. Roque, the eastern point of South America in the 
map for January, lying within the 80° isotherm, one can only guess that 
