8 ON THE PARALLEL ROADS 



teen inch levelling instrument, made by Jones. It was plant- 

 ed, as nearly as we could guess, (where the limits of the sub- 

 ject we had to work upon were so ill defined,) rather towards 

 the lower edge of the shelf, and we endeavoured to adjust it in 

 such a manner, as to make the levelling telescope itself of the 

 same height with what we supposed to be the higher or inte- 

 rior angle of the shelf. Directing the object-glass of the in- 

 strument to the nearer and immediately opposite correspond- 

 ing line of shelf, it applied all along, most accurately, to the 

 horizontal hair ; but when pointed to those farther off, (some 

 of which were perhaps five or six miles distant,) they appear- 

 ed to sink sensibly below the hair, and this in proportion to 

 their distance from the point where we stood ; but they were 

 nowhere observed to do so in a greater ratio, than the allowance 

 for the curvature of the earth at such rectilineal distances de- 

 manded. And, what was in our opinion most conclusive, 

 when the telescope was pointed to, and made to traverse along 

 any particular portion, which, from being directly opposite to 

 the eye, might have been presumed to be nearly equidistant 

 in all its parts, it was found to preserve an uniform relation to 

 the horizontal hair. 



Our observation taken on shelf 3d, was, if possible, even 

 more satisfactory, from the point we chose, (see/' in the map,) 

 being still better adapted for the purpose, as we not only com- 

 manded a view of the head of what may be termed Lower 

 Glen Roy, (see plate V., the sketch for which was taken from 

 hence,) but we could also see the shelves, running continuous 

 all the way down to the mouth of Glen Turret, to which we 

 were opposite. We could follow them with the eye throughout 

 the greater part of their progress around that tributary glen, 

 and so down the north-west side of Glen Roy, for a con- 

 siderable distance ; forming altogether a line, which, if it had 



been 



