As the varying dimensions of the valley cannot be easily computed, it is unnecessary to examine the question with hydro-statical accuracy: a general inquiry as to the probable effects will be sufficient.
A general notion of the difference of capacity of the different parts of Glen Roy, may be acquired from comparing its vertical and lateral dimensions at its extremities; [1] and for the sake of simplicity I shall limit myself to this single case, although it is obvious that many more complicated considerations must enter into the account. It will also be most simple to assume the lowermost line as the groundwork of this investigation, as we here get a minimum ratio. This cuts the rocky bottom of the valley near its uppermost extremity.[2] A point therefore exists in the valley where its vertical dimension, considered from its bottom to the summit of the lowermost line is nothing, its breadth being in that place about half a mile. If now we examine its lower extremity, or that part of the joint valley of the Roy and Spean, where the last point of the same line is visible, we find it situated at a considerable height (not less than 800 feet) above the bottom of the valley, while its breadth is in this place five miles. The simple statement of this fact renders it superfluous to say that a body of water could not flow through a valley of this form, with a level surface; a condition required to produce the line in question. It is plain that the same objection applies to the case of the two upper ones, although with somewhat less force, according to the ratios of the several inequalities at their upper and lower extremities.
As connected with this last argument we may now ask, by what system of operations, on the same hypothesis, the lines were traced
