171 



A Table is given showing the descent for every day of that period, 

 from 7 A.M. to 6 P.M. and from 6 P.M. to 7 A.M. 



In the months when there was no sunlight from 6 P.M. to 7 A.M., 

 there was no descent in that interval. The descents from 7 A.M. to 

 6 P.M. were very different on different days. Sometimes they 

 amounted to a quarter of an inch in the day, and sometimes were 

 not appreciable. The greatest descents were on sunny days, and 

 especially when with a warm sun there was a cold wind. The least 

 were on days of continual rain. The average daily descents were, in 

 inches, 



These descents were not due to the extreme temperatures of the 

 periods in which they took place, but to the aggregate of the variations 

 up and down during each interval. The difference of the highest and 

 lowest temperatures in any interval may have been small, and yet the 

 changes of temperature up and down may have been many, and their 

 aggregate great. It is upon this aggregate that the descent depends. 



The dilatation of ice was measured in the years 1845, 1846, at the 

 Observatory of Pultowa, by Schumacher, Pohr, and Moritz ; and the 

 particulars of their experiments were communicated to the Academy 

 of St. Petersburgh, by "W. Struve, in 1848, and published in its 

 Memoirs (Sciences Mathem. et Phys., ser. 6. t. iv.). By exposing 

 water to the action of the frost in a mould, Schumacher obtained a 

 block of ice, which, after reducing it with the plane, measured 6 ft. 

 3 ins. in length and 6 ins. by 6^ inches in section ; and he caused three 

 thermometers to be frozen into it with their stems projecting above 

 its surface. This block of ice he carried out from a room, where it 

 had been preserved at a uniform temperature of 2 R. during the 

 day, into the open air at night, and slung it in a horizontal position 

 from a beam supported by tressles. As its temperature fell he 

 measured the distance between two steel points frozen into it near 

 its two ends, by a measuring rod of dry wood (well-clothed), the di- 

 stances on which were referred to a standard measure on the wall of 

 a room of the Observatory which retained nearly a constant tempe- 



