TEMPERATURE 69 
Readings 
99. Time. The hourly and daily fluctations of the temperature of the 
air render frequent readings desirable. It is this variation, indeed, which 
makes single readings, or even series of them, inconclusive, and renders 
‘the use of a recording instrument almost imperative in the base station at 
least. Undoubtedly, a set of simultaneous readings at different heights in 
one. station, or at the same height in different stations, especially if made 
at the maximum, have much value for comparison, but their full significance 
is seen only when they are referred to a “continuous base record. Such 
series, moreover, furnish good results for purposes of instruction. In re- 
Fig. 21. Richard thermograph. 
search work, however, it has been found imperative to have thermographs 
in habitats of widely different character. With these as bases, it is possible 
to eke them out with considerable satisfaction by means of maximum- 
minimum thermometers in less different habitats, or in different parts of 
the same habitat. Naturally these are less satisfactory, and are used only 
when expense sets a limit to the number of thermographs. In a careful 
analysis of a single habitat, more can be gained by one base thermograph 
supplemented by three pairs of maximum-minimum thermometers in dissimi- 
lar areas of the habitat than by two thermographs, and the cost is the 
same. eG 
