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NEW YORK STATE MUSEUM 



can be divided, approach the mean, but rather what are the ex- 

 treme divergencies from the mean of such shorter periods. By 

 this method, any approach to a general law will soon be detected, 

 if the extreme divergencies are found gradually to decrease, as 

 the subordinate periods into which the record has been divided 

 increase in length. 



Also, What is the probable accuracy of any record the length of 

 which is less than that necessary to give an average which will 

 not be materially altered when the record is extended? 



Space will not be taken to show Mr Binnie's views in detail, 

 for which reference may be made to the abstract in the second 

 Hudson report, or, for the complete views, to the paper in the 

 Proceedings of the Institution of 'Civil Engineers, but assuming 

 that the observations are properly made it is stated that " depend- 

 ence can be placed on any good record of thirty-five years' dura- 

 tion to give a mean rainfall correct within 2 per cent of the 

 truth." 



Further, it can be stated that for records from twenty years to 

 thirty-five years in length, the error may be expected to vary 

 from 3.25 per cent down to 2 per cent, and that for the shorter 

 periods of five, ten, and fifteen years, the probable extreme devia- 

 tion from the mean would be 15 per cent and 4.75 per cent, 

 respectively. 



A twenty years' record, therefore, may be expected to show an 

 error of 3.24 per cent. This is about as close as rainfall records 

 in this country will agree, as comparatively few are much beyond 

 twenty years in length. 



In his paper on Rainfall of the United 'States, Mr Henry has 

 examined this question, using long records at New Bedford, St 

 Louis, Philadelphia, Cincinnati, and other places. The rainfall has 

 been measured at New Bedford for 83 consecutive years, and at 

 St Louis for 60 years. For a 10-year period Mr Henry found 

 the following variations from the normal : At New Bedford + 16 

 per cent and — 11 per cent; at Cincinnati, + 20 per cent and — 

 17 per cent; at St Louis, 4- 17 per cent and — 13 per cent; at 

 Fort Leavenworth, + 16 per cent and — 18 per cent; and at San 



