Page:Popular Science Monthly Volume 3.djvu/766

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 blameless 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 describe 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 references 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 continue 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 the 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, respectively.

"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 measurements in subsequent years. His researches are recorded in a work entitled 'Système 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 various 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.'