1845.] UNFAVOURABLE CONDITIONS OF WEATHER. 113 



The results of these observations will be best understood 

 by consulting the projected results in Plate IX. fig. 2 [of the 

 Philosophical Transactions for 1846],* in which the advance 

 of the marks is magnified twofold relatively to the spaces 

 between them, which was necessary for the reduced scale of 

 the engraving, although it has the disadvantage of increasing 

 the deviations from symmetry, which might arise from errors 

 of observation. As those who have not attempted the actual 

 execution of such experiments cannot be aware of the difficulties 

 they entail, it may be just to mention, that during weather so 

 unfavourable as that which occurred during the continuance of 

 these experiments, nothing can be so irksome as the necessity 

 of persevering in the face of physical obstacles, the only alterna- 

 tive which the necessary limitation of my stay afforded being to 

 abandon them. I do not speak of the painful effort of conduct- 

 ing delicate observations for hours under a hot sun, whilst the 

 feet are immersed in the liquid sludge of decaying snow, for I 

 am not aware of having sacrificed the precision of a single 

 observation to such a cause (though in the course of my glacier 

 experience I have sometimes been compelled to abandon or dis- 

 continue observations), but it is easy to see that the success of 

 experiments like this depended upon the absolute fixity of the 

 marks inserted in the unstable and wasting surface of the gla- 

 cier, and that the most dry and uniform condition of the ice 

 seemed alone to promise a chance of finding the small pins in 

 the exact positions in which they had been planted a day or 

 two before. Instead of this, eleven out of nineteen days which 

 I spent at Chamouni were wet, and notwithstanding the sea- 

 son of the year, the glacier was repeatedly covered with snow, 

 which, in melting under a succeeding fierce sun, left the surface 

 honeycombed by infiltration, and streaming with wet, so that 

 the preservation of the holes was only effected by laboriously 

 covering every one with large flat stones during the intervals of 

 observation, and even this was not free from other disadvantages 

 which it would take too long to particularize. On the whole, 



* [See note to page 109, above.] 



