72 Meteorology. 



that dry weather, durhig Summer, is also in general very warm and op- 

 pressive, for want of the cooling influence of evaporation and vice versa, 

 moist Summers are generally cool. 



The severity of the weather, therefore, at the close of the year, 

 seems to have been but a compensation for the unusual warmth, which 

 had prevailed over nearly the whole of the preceding portion, so that a 

 mean temperature of 51.5° resulted, which is not more than a degree 

 higher than the mean temperature of the preceding six years. Another 

 law of the weather here presents itself to us, viz : that extremes of cold 

 or heat, or of any other kind of weather, are, in the course of several 

 months, compensated by the prevalence of an opposite condition of 

 atmosphere, so that a mean result is at length produced. This is one 

 of those beautiful and wise arrangements of the Creator, which are to 

 be found in every part of the physical world. 



2. The past year was also more than usually deficient in moisture, 

 as will be seen by a reference to the following table ; the mean or aver- 

 age for the last seven years being 37.206 inches, whilst the quantity for 

 1845 was only 30.19 inches : 



1839 1840 1841 1842 1843 1844 1845 Mean. 



Inches, 38.012 31.771 45.134 36.238 47.631 31.167 30.19 37.206 



When it is stated that the moisture deposited during the year is so 

 many inches, the meaning is, that if all the rain and all the snow if 

 melted, which fell during that period, were to fall at once it would cover 

 the earth to that depth. The rain is caught as it falls in a vessel ex- 

 posed to the open air day and night, and afterwards measured in a grad- 

 uated tube, of which a cross-section is equal in area to the one-hun- 

 dredth part of that of the vessel. The snow is melted and then meas- 

 ured, or, which is found by repeated trials to give the same result, if 

 the snow be not more than ordinarily moist, ten inches of snow are es- 

 timated as equivalent to one of rain. 



It must not be supposed that 37.2 inches, which is the mean quan- 

 tity of rain at Gettysburg, is also the mean for every other place. The 

 annual quantity of moisture is a very variable item, being very much in- 

 fluenced by local circumstances, but in general diminishing with the in- 

 crease of latitude. In some localities in the equatorial regions the 5'early 

 fall is more than 120 inches or 10 feet. 



The whole amount of snow which fell from the first of January un- 

 til April was 31.37 inches, whilst that of December alone was 23.5 

 inches, showing a considerable deficiency in the beginning of the year ; 

 a partial compensation having however been produced by several inches 

 of rain, which fell during that period. 



