352 Mr. Nixon on the Measuremeni {by Trig07iometry) of the 



other signals consisted of conical piles about twelve feet in 

 diameter at the base, and seven or eight feet high, constructed 

 externally of huge sods, and filled up with loose peat-earth. 

 There exists, however, this gi-eat objection to a turf-signal 

 placed on a peat moor, — that the observer may not be able to 

 distinguish it, at a remote station, from the peat stacks with 

 which it is liable to be surrounded. For instance, although 

 the pile of sods marking the most elevated point of the ex- 

 tensive and nearly level summit of Penhill, famous for its ex- 

 cellent turbaries, was placed on the northern extremity of an 

 elevated cam (or mound) dividing Walden from Coverdale; 

 yet so numerous were the heaps of peat raised from time to 

 time to the west and north, that the signal, when observed 

 from stations in those directions, could seldom be satisfactorily 

 identified ; — an uncertainty which has led to unusual discre- 

 pancies in the measurements of such of the distances as relate 

 to that signal. 



The horizontal angles were, with one exception*, measured by 

 the six-inch theodolite, minutely described in Phil. Mag. and 

 Annals, vol. iii. pp. 83 — 86. At Bear's Head, Shunnor Fell, 

 BakestoneEdge, and Penhill, immediately on completing one set 

 of observations, the telescope was reversed in position within its 

 Ys, and the observations, notwithstanding the consequent in- 

 convenient position of the tangent screws, carefully repeated. 

 From the unsettled state of the atmosphere, it would frequendy 

 occur that some of the distant hills had been obscured by mist 

 or haze during the course of either the first or second set of 

 observations, which rendered it necessary to apply to their 

 readings the uncertain correction of half the mean difference 

 of the two readings, registered for observations with the tele- 

 scope in both positions. At Settronside, the summits of se- 

 veral of the fells were so rapidly shrouded in vapour that no 

 time was afforded to reverse and re-adjust the telescope ; but 

 a few of the observations of most importance were repeated 

 (by turning the screw situated under the parallel plates) on 

 different parts of the divided circle. 



It has been remarked, that the signals of the preceding sur- 

 vey, when observed from remote stations, althougli viewed 

 under an angle considerably greater than that subtended by 

 the fine vertical wire of the telescope, would totally disappear 

 on interposing the latter; but the signals just described, from 

 their superior magnitude, could be bisected in tolerably clear 

 weather with the greatest ease and accuracy. On the occur- 

 rence, however, of haze in the atmosphere, the signal, suffi- 



* The angle at Rover Crag between Harlen Fell and Penhill. 



ciently 



