748 THE POPULAR SCIENCE MONTHLY. 



been thought best for the present to meet them, I must leave my friend 

 and fellow-laborer, Prof. Tait, to tell." 



The book here referred to is the unpretending volume whose blame- 

 less advent I have just described. 



I have not the honor of knowing Principal Shairp personally, but 

 he will, I trust, permit me to assure him of two things : Firstly, that, 

 in writing my book, I had no notion of rekindling an extinct fire, or 

 of treating with any thing but tenderness the memory of his friend. 

 Secondly, that, had such been my intention, the negative attribute, 

 " not obscure," is hardly the one which he would have chosen to de- 

 scribe the words that I should have employed. But the fact is, the fire 

 was not extinct : the anger of former combats, which I thought spent, 

 was still potential, and my little book was but the finger which pulled 

 the trigger of an already-loaded gun. 



Let the book speak for itself. I reproduce here in extenso the ref- 

 erences to Principal Forbes, which have been translated into "charges" 

 against him by Principal Shairp. Having, in section 20, mentioned 

 the early measurements of glaciers made by Hugi and Agassiz, I con- 

 tinue thus : 



" We now approach an epoch in the scientific history of glaciers. Had the 

 first observers been practically acquainted with the instruments of precision 

 used in surveying, accurate measurements of the motion of glaciers would 

 probably have been earlier executed. We are now on the point of seeing such 

 instruments introduced almost simultaneously by M. Agassiz on tbe glacier of 

 the Unteraar, and by Prof. Forbes on the Mer de Glace. Attempts had been 

 made by M. Escher de la Linth to determine the motion of a series of wooden 

 stakes driven into the Aletsch Glacier, but the melting was so rapid that the 

 stakes soon fell. To remedy this, M. Agassiz, in 1841, undertook the great 

 labor of carrying boring-tools to his ' hotel,' and piercing the Unteraar Glacier 

 at six different places to a depth of ten feet, in a straight line across the glacier. 

 Into the holes six piles were so firmly driven that they remained in the glacier 

 for a year, and, in 1842, the displacements of all six were determined. They 

 were found to be 160 feet, 225 feet, 269 feet, 245 feet, 210 feet, and 125 feet, re- 

 spectively. 



" A great step is here gained. You notice that the middle numbers are the 

 largest. They correspond to the central portion of the glacier. Hence, these 

 measurements conclusively establish, not only the fact of glacier motion, but 

 that the centre of the glacier, like that of a river, moves more rapidly than the 

 sides. 



" With the aid of trained engineers, M. Agassiz followed up these measure- 

 ments in subsequent years. His researches are recorded in a work entitled 

 ' Systeme Glaciaire,' which is accompanied by a very noble Atlas of the Glacier 

 of the Unteraar, published in 1847. 



" These determinations were made by means of a theodolite, of which I will 

 give you some notion immediately. The same instrument was employed the 

 same year by the late Principal Forbes upon the Mer de Glace. He established 

 independently the greater central motion. He showed, moreover, that it is not 

 necessary to wait a year, or even a week, to determine the motion of a glacier ; 

 with a correctly-adjusted theodolite he was able to determine the motion of va- 

 rious points of the Mer de Glace from day to day. He affirmed, and with truth, 

 that the motion of the glacier might be determined from hour to hour. We 

 shall prove this farther on. Prof. Forbes also triangulated the Mer de Glace, 

 and laid down an excellent map of it. His first observations and his survey 

 are recorded in a celebrated book published in 1843, and entitled 'Travels in 

 the Alps.' 



