510 POPULAR SCIENCE MONTHLY. 



when the barometers or thermometers were above or below certain 

 marks, and so on. 



In dealing with any complicated problem it is usually best to try 

 and simplify it as much as possible, and if the problem even then ap- 

 pears too difficult, to attack a special case, and this is undoubtedly 

 the best plan to pursue here. In order to avoid the rapid fluctuations 

 which occur in the temperate zones, meteorologists have turned to 

 places in the tropics where the types of weather during the course of a 

 year are apt to be much more permanent and where the changes from 

 one year to another are likely to appear more clearly than in other 

 regions of the earth. It seems to be generally agreed that, owing to its 

 political and geographical situation, India is the country best fitted to 

 satisfy these various conditions. The most marked peculiarity of its 

 climate, taking the country as a whole, is a rainy season extending over 

 some two or three months, with comparatively little precipitation per 

 month during the rest of the year. Independently of the scientific 

 side of the question, the quantity of rainfall and the duration of the 

 rainy season are of immense practical importance, since a deficiency in 

 either may and usually does involve a famine. An examination of 

 Indian records may therefore be of great value; and as a fairly con- 

 tinuous series of observations at several observatories has been obtained 

 for some twenty or thirty years, it may not be too early to commence 

 a systematic examination of them. 



Several investigations in this direction have been made lately, un- 

 fortunately with but little success in obtaining positive results. Here 

 it is advisable to set forth with a little more detail what is meant 

 by ' success,' since it is necessary to have a clear conception of the 

 value, both scientific and practical, to be attached to the results of 

 investigations based on the theory of averages. A set of observations 

 is recorded and an examination of them is made with a view of finding 

 out the existence of a cycle. Suppose that one cycle is found, but that 

 its effect on any individual observation is very small. For example, 

 if the temperatures at any particular place vary in the course of a year 

 from 20° to 90° and we find a cycle running through all its changes 

 in eleven years and having a maximum effect of 1°, the daily tem- 

 peratures are scarcely altered if we subtract the amount corresponding 

 to this long-period variation. Thus, though the cycle may be thor- 

 oughly well made out, it tells very little about the variations of tem- 

 perature. It is only when we are able to get a sufficient number of 

 periodic changes whose combined effect will fairly well represent the 

 individual observations that we can be said to be at all successful in 

 our analysis of the latter, and it is only then that a basis is found 

 sufficient to predict the future numbers of the series. 



Of the various attempts to trace long-period changes in the 



