222 DISCUSSION OF THE DESICCATION QUESTION. 



temperature on the earth since civilization became so far advanced that his- 

 torical records of the movements of the human race began to be preserved. 



The first source, in this connection, to ^vhich the inquirer would natu- 

 rallj' look for information would be the records of instrumental observations 

 taken all over the world. In the present stage of meteorological science we 

 should expect to find evidences of climatic change, as recorded by instru- 

 mental measurements, in one or both of two conditions, variations in the 

 mean temperature and in the mean amount of precipitation. The varia- 

 tions in the direction and force of the wind form an important element of 

 climate, but observations of this kind have not been taken with accuracy 

 until within a few years, the machinery for that purpose being somewhat 

 complicated and expensive. 



It might be supposed that among the proofs of desiccation over the earth's 

 surface the results of instrumental observations indicating such decrease of 

 rain-fixll would have been brought forward. This has not been done ; but it 

 must not be inferred that a drying-up could not have been going on, because 

 it cannot be proved by recorded observations of the rain-gauge to have done 

 so. Neither should we be justified in assuming that an absence of proof of 

 this kind in reference to precipitation would be sufficient reason for not 

 searching for instrumentally recorded evidence of a change of temperature. 

 The conditions in regard to determinations of mean temperature and of 

 mean precipitation are considerably different, as will be briefly explained. 



Variations from year to year in the mean which would result from a long 

 series of observations are for the same region or country often very great, 

 even over areas where the precipitation is quite large ; and in regions where 

 it is very small, this irregularity is still more striking. Thus at San Fran- 

 cisco, between the years 1850 and 1872, the annual amount of rain ranged 

 between 7-4 and 49.27 inches. In parts of Lower California, on the other 

 hand, it is stated that rain .sometimes does not fall at all for several years, 

 but that there are seasons in which several inches are furnished. Evidently 

 in such cases a series of observations extending over many years would be 

 required before what could be fairlj' called a true mean would be obtained. 



There is also another difficulty connected with the determination of the 

 mean amount of tlie precipitation over any considerable area, namely, that 

 its distribution is locally so irregular. An examination of the rain-charts 

 of England will illustrate what is here meant, for it will be seen at once 

 that places but a few miles removed from each other show very great 



