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monthly means are insufficient; and the necessity has been felt 

 of computing the mean temperatures for periods of much shorter 

 duration. The Meteorological Institutions of those of the European 

 States which have taken the foremost part in the prosecution of 

 meteorology, have in consequence adopted five-day means, as the 

 most suitable intermediate gradation between daily and monthly 

 means : and as an evidence of the conviction which is entertained 

 of the value of the conclusions to which this investigation is likely 

 to lead, it has been considered worth while to undertake the pro- 

 digious labour of calculating the five-day means of the most reliable 

 existing observations during a century past. This work is already 

 far advanced ; and it cannot be too strongly recommended, that at 

 all fixed stations, where observations shall hereafter be. made with 

 sufficient care to be worth recording, five-day means may invariably 

 be added to the daily, monthly, and annual means into which the 

 observations are usually collected. The five-day means should always 

 commence with January 1, for the purpose of preserving the uni- 

 formity at different stations, which is essential for comparison : in 

 leap years, the period which includes the 29th of February will be 

 of six days. 



In treating climatology as a science, it is desirable that some 

 correct and convenient mode should be adopted, for computing and 

 expressing the comparative variability to which the temperature in 

 different parts of the globe, and in different parts of the year in the 

 same place, is subject from non-periodic causes. The probable 

 variability, computed on the same principle as the probable error of 

 each of a number of independent observations, has recently been 

 suggested as furnishing an index " of the probable daily non-periodic 

 variation " at the different seasons of the year ; and its use in this 

 respect has been exemplified by calculations of the " index " from 

 the five-day means of twelve years of observations at Toronto, in 

 Canada (Phil. Trans. 1853, Art. V.). An index of this description 

 is of course of absolute and general application ; supplying the 

 means of comparing the probable variability of the temperature in 

 different seasons at different places (where the same method of com- 

 putation is adopted) as well as at the same place. It is desirable that 

 this (or some preferable method if such can be devised for ob- 

 taining the same object) should be adopted by those who may desire 



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