INVESTIGATION OF TEMPEKATUEE CHANGES. 223 



discrepancies in the mean rain-fall, and it is hardly possible to suppose that a 

 trustworthy result could be had for this country, which forms so small a por- 

 tion of the entire land area of the globe, w ithout occupying some hundreds 

 of stations. Thus it seems evident that a great number of observers whose 

 work should have been continued over a very long series of years would be 

 an indispensable requisite to the determination of a small secular variation in 

 the precipitation over an^^ region of considerable extent. Neither would it 

 be safe to base any conclusions as to general climatic change on observations 

 which embraced only a small area of land surface, for we know not how for 

 small local fluctuations miglit be compensated by others of an opposite char- 

 acter at some not far distant point. 



Tile difficulty of obtaining the mean temperature of any particular locality 

 is much less than that of fixing the mean amount of precipitation. At a 

 certain depth, the amount of which is dependent chiefly on the annual range 

 of the temperature, but which is nowhere very great, we come to a jjoint 

 where the thermometer remains stationary at a figure indicating the mean 

 tempei'ature of the year at the surface at that locality. It is true that ther- 

 mometric observations are not usually taken below the surface, in the region 

 of invariable temperature, because it is the fluctuations from liour to hour, 

 day to day, and season to season, which it is desirable, for ordinary purposes, 

 to know, while the secular variation is a matter of much less immediate 

 consec|uence. Such subterranean observations have been made, in one local- 

 ity at least, as will be mentioned forther on. 



Accurate records of either temperature or rain-fall do not, however, in any 

 place extend back for any consideraljle time : that is to sa}', in (comparison 

 with what would be desirable, in order that a positive result should be 

 reached in reference to the question before us. 



It is true that all effbrts to translate geological into historical time have failed 

 to give any satisfactory result, so that we hd^e no definite idea of how many 

 years would be a reasonable term to fix on as likely to be sufficient for the 

 accomplishment of a change of climate great enough to become decidedly 

 perceptible to accurate instruments of record. Certainly most geologists 

 would admit that a hundred years was a very short period — from a geo- 

 logical point of view — for this purpose. If there had been a distinct dimi- 

 nution of temperature in one lumdred years, then — as it appears to the 

 writer, at least — the change would seem to the geologist to be a rapid one, 

 however slow it might be considered by the general public. 



