714 PHILOSOPHICAL TRANSACTIONS. [aNNO 1775. 



from the 2 ends of this side, and a long slant pole from the opposite angle. Two 

 sliding arms were put on the upright pole, capable of being raised or depressed, 

 one above and the other below the place where the slant pole was fastened to the 

 upright pole, for supporting the measuring poles at a convenient height above 

 the ground. In measuring the base, one end of a pole was supported on one 

 of the stands, and the other end on another stand ; and it was set horizontal 

 by means of a spirit level laid on it about the middle, and by raising or depress- 

 ing the arm on which it rested at one or the other end. The other pole was 

 then, in like manner, supported on the 2 other stands truly level, and in the 

 same vertical plane with the former pole, namely, that of the intended base 

 without regarding whether they were exactly of the same height, and with some 

 small horizontal interval between their ends. This interval was measured by 

 laying one leg of a brass rectangle, which was divided into inches and tenths, 

 along one pole, while the other, or vertical leg, touched the end of the other 

 pole ; for it was not thought advisable, to bring the ends of the poles to touch 

 exactly, as that would have taken up a great deal of time, and might have 

 endangered the altering the position of the hindermost pole, if it should chance 

 to receive any shock by laying down the foremost pole. It is evident that the 

 inches and tenths given by the divisions of the brass rectangle are to be added 

 into one sum together with the poles, in computing the length of the base. 

 When the foremost pole was truly placed, and the interval between them had 

 been measured by the divided side of the brass rectangle, the hindermost pole 

 was taken up, and the stands on which it had rested were advanced forwards, 

 and the pole again laid on them, truly level, and in the true direction of the 

 base. In order to set the poles continually in the proper direction of the base, 

 the following method was used. The theodolite was first set up at one end of 

 the base, and an upright pole at the other, and another in the middle, and a 

 third was from time to time advanced to a little distance forward ; and the mea- 

 suring poles were sometimes placed in the proper direction by the eye, looking 

 along the lengths of both poles together to the upright pole before them, and 

 sometimes by the help of the theodolite. In this manner, about the middle 

 of September, a base was measured by Mr. Burrow and Mr. Menzies of 3012 

 feet, in the valley at the foot of the hill to the south west ; but not so accurately 

 as this method is capable of, owing to the stands being very unsteady, through 

 the looseness of the spikes in the feet and other faults, during the measuring the 

 first quarter of the base, though they were mended before the mensuration of 

 the remainder of it. The mensuration of another base of the length of 5807 

 feet, in the meadow of Rannoch, about 24. miles to the north-west of the 

 centre of the hill, which I attended tayself, was performed with the greatest 

 accuracy, according to the same method, on the J 0th, 11th, and 12th of Oct., 

 with new stands, more substantial and firm than the former. 



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