CLOUDS AND RIVERS, ICE AND GLACIERS. 65 



tioti and carefully levelled. The telescope is directed 

 6rst upon tlie standard point at the opposite side of 

 the valley, being moved by a tangent screw until the 

 intersection of the spider's threads accurately covers 

 the point. The telescope is then lowered to the 

 first stake, beside which our trained assistant is 

 already standing. He is provided with a staff with 

 feet and inches marked on it. A glance shows us 

 that the stake has moved down. By our signals the 

 assistant recovers the point from which we started 

 yesterday, and then determines the distance from this 

 point to the stake. It is, say, 6 inches ; through 

 this distance, therefore, the stake has moved. 



160. We are careful to note the hour and minute at 

 which each stake is driven in, and the hour and the 

 minute when its distance from its first position is 

 measured; this enables us to calculate the accurate 

 daily motion of the point in question. The distances 

 through which all the other points have moved are 

 determined in precisely the same way. 



161. Thus we shall proceed to work, first making 

 clear to our minds what is to be done, and then making 

 sure that it shall be accurately done. To give our work 

 reality, I will here record the actual measurements 

 executed, and the actual thoughts suggested, on the 

 Mer de Glace in 1857. The only unreality that 1 

 would ask you to allow, is that you and I are supposed 

 to be making the observations together. The labour 



