1846.] 



ABLATION AND SUBSIDENCE MEASURED. 



171 



vertical height. The height of the eye above or below the fixed 

 point was measured, and the sum or the difference (as the case 

 might be) of this measure and the last gives the difference of the 

 level of the horizontal hole in the ice, and the mark on the 

 moraine. The following may serve as an example : 



STATION U, NEAR MONTANVERT. 



Of course the horizontal hole may be renewed as often as 

 convenient, all that is necessary being to ascertain the difference 

 of level of the old and new hole. 



In the preceding example (which has been selected by 

 chance), the subsidence bears an unusually small proportion to the 

 ablation. At the station in question, the average daily ablation 

 in July and August was 3*62 inches, the average daily subsidence 

 1*63 inches. The sum of the two, or the geometrical depres- 

 sion of the surface 5'25 inches, whereof seven-tenths were pro- 

 duced by ablation, three-tenths by subsidence. These relations, 

 together with those opposite the old station Q, are shown in one 

 view in the following table of 



MEAN EESULTS. 



The last two columns show the effects of the ablation and 

 subsidence- in hundredth parts of the whole depression. 



* See Plate VI. fig. 1. 



f Taken from the observation of the neighbouring mark D'2. 



