EXPLANATION OF TABLE (A). 



The headings of the different columns of the table, in general, sufficiently explain 

 their contents, but a few additional remarks may be made, principally relating to 

 the position of the stations and to the continuity and combination of records. 



The latitude and longitude of many stations near the sea-coast, and especially 

 of the military posts there situated, are given on the authority of the U. S. Coast 

 Survey ; positions near the Great Lakes are from the survey of the lakes by officers 

 of the U. S. A. ; other positions and all those in the interior are taken from the 

 best sources available, and frequently depend on the authority of the observers. 

 In the absence of any trigonometrical survey of the States (excepting Massachu- 

 setts) and Territories, these assigned geographical positions are subject to much 

 uncertainty; that in latitude may reach ±5' and that in longitude ±12' for the 

 more extreme cases, and may reach half these amounts in ordinary cases. Pro- 

 bably few will be found exceeding these limits. 



The height of the place of observation above the ocean is given on the authority 

 of the observer, unless corrected from other and apparently more reliable sources. 

 These data are known in many instances to be quite defective, but there are now 

 no means at hand to improve them. 



The tabular means given for any one month are derived from all monthly 

 amounts in the different years, and, unless the whole series of observations is 

 unbroken, may differ in weight, since they may be derived from a greater or less 

 number of years, and, for the same reason, of discontinuity in the observations; the 

 numbers in the column headed " Extent" do not always correspond to the time 

 elapsed from the date of commencement to that of the close of the series, as given 

 in the column following. The column headed " Extent" shows the number of 

 years and months of actual observation. The contents of the table is generally 

 brought up to the year 1867. 



"When more than one series for any limited locality was on record, either cover- 

 ing the same period or for different years, consolidated results are given at the 

 bottom of the table, at least for the more prominent localities. For these consoli- 

 dated results, the several monthly values observed for any one month and for any 

 one year were first combined by taking the mean, and these monthly means for the 

 several years were afterwards united. This mode of combination must necessarily 

 be followed on account of the annual inequality in the rain-fall. The observer's 

 name is stated, except for the series of observations taken by the U. S. Army, and 

 for those taken by the New York University system, and necessarily for a few 

 other stations where the observer's name was not given. The observations of the 

 army were generally taken by the assistant surgeon of the post, and those of the 



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