153. 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 Professor 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 (162). Professor 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." 154. These observations were also followed up in subsequent years, the results being recorded in a series of detached letters and essays of great interest. These were subsequently collected in a volume entitled "Occasional Papers on the Theory of Glaciers," published in 1859. The labours of Agassiz and Forbes are the two chief sources of our knowledge of glacier phenomena. § 21. _The Theodolite and its Use. Our own Measurements._ 155. My object thus far is attained. I have given you proofs of glacier motion, and a historic account of its measurement. And now we must try to add a little to the knowledge of glaciers by our own labours on the ice. Resolution must not be wanting at the commencement of our work, nor steadfast patience during its prosecution. Look then at this theodolite; it consists mainly of a telescope and a graduated circle, the telescope capable of motion up and down, and the circle, carrying the telescope along with it, capable of motion right and left. When desired to make the motion exceedingly fine and minute, suitable screws, called tangent screws, are employed. The instrument is supported by three legs, movable, but firm when properly planted. 156. Two spirit-levels are fixed at right angles to each other on the circle just referred to. Practice enables one to take hold of the legs of the instrument, and so to fix them that the circle shall be nearly horizontal. By means of four levelling screws we render it _accurately_ horizontal. Exactly under the centre of the instrument is a small hook from which a plummet is suspended; the point of the bob just touches a rock on which we make a mark; or if the earth be soft underneath, we drive a stake into it exactly under the plummet. By re-suspending the plummet at any future time we can find to a hairbreadth the position occupied by the instrument to-day.