-1859] WRITINGS OF JOSEPH HENRY. 211 



will exhibit a lower temperature than one under the same 

 material at the surface of the ground. This effect however 

 is not entirely due to the screening influence of the covering, 

 but in part to the fact that the intense rays of the heat of 

 the sun as well as those of the light of the same body penetrate 

 the crystals of the snow as they do the glass covering of a 

 hot-house, and being absorbed by the dark ground beneath 

 elevate the temperature. For the same reason in bright 

 days the snow next to the slate roof of a house is seen to 

 melt, while the upper surface remains unaffected. 



There is a singular phenomenon observed during the 

 spring of the year in damp, sandy places, which has attracted 

 much attention, namely, the ice-columns which spring from 

 the earth during cold nights, elevating small gravel-stones 

 on their tops, and raising as it were above its usual level the 

 general surface of the ground. These crystals have been 

 carefully studied by Professor John Le Conte, and appear to- 

 be due to the law we have before mentioned of the axis of 

 crystallization being always at right angles to the surface of 

 cooling, as well as to the attraction of the water for itself and 

 the consequent excluding effect of all extraneous bodies. 

 The water of which these crystals are formed is drawn up 

 from below by capillarity ; is frozen as it comes up to the 

 surface in vertical prismatic crystals ; a new portion is drawn 

 between the basis of the crystals first formed and the ground, 

 which is also frozen ; and so the process is continued until 

 stopped by the failure of moisture, or the increase of the 

 temperature due to the advancing heat of the day. 



The next subject in order of which we intended to treat 

 is that of the vapor of water in the atmosphere ; but this is 

 of so important a character in its connection with all the 

 phenomena of the fitful changes of the weather, and the 

 peculiarity of climate, as well as with the agricultural prod- 

 ucts of a country, that justice cannot be done to it within 

 the limits assigned to meteorology in this Report, and there- 

 fore we shall defer it until next year. * 



* [Forty-three pages of Meteorological Tables following this part are 

 omitted in the present re-print.] 



