6 Dr. H. Schlagintweit's Observations in the Alps 



C. Insolations observed contemporaneously on Johannishiitte and 

 on the Glacier {horizontal distance from the edf/e 900 P. F.). 



The differences can in general be very considerable; even on 

 the higher alpine summits (Table A.), notwithstanding the low 

 temperature of the air, they are always very appreciable. A dif- 

 ference of 2° - 5 is frequently observed even there, and on very 

 favourable days it amounts even to 6° C. The experiments upon 

 the Johannishiitte show that the insolations increase towards 

 midday, but many disturbances are at the same time observed 

 during the process of experiment. We therefore limit ourselves 

 to the results of the hours, with the remark, that the observa- 

 tions were made on days widely separated from each other. The 

 Table C. includes observations, every two of which contained 

 between the braces are correspondent ; the upper one was made 

 on the Johannishiitte, the other at the same time upon the gla- 

 cier. The temperatures of the shadowed atmosphere are very dif- 

 ferent at both points ; the height of the thermometer in the sun 

 is, however, in both cases very nearly the same. A disturbance 

 due to the reflexion of light from the surface of the glacier might 

 in the present case be expected ; but this was prevented by the 

 introduction of a pasteboard screen beneath. The difference of 

 the sunned and shaded thermometers is here always greater than 

 that observed over the rocks which surround the glacier ; at noon 

 the difference in the former case exceeds that in the latter by 

 1°*46 C. This deviation is due to the circumstance of our 

 having in one case an atmosphere artificially cooled by the pecu- 

 liar surface upon which it rests. Both thermometers indeed 

 are hereby depressed ; the difference, however, must be greater 

 on the glacier, as the intensity of the solar rays, in the observa- 

 tions made contemporaneously at both places, is the same, and 

 is therefore more appreciable in the relatively cold atmosphere. 



