of Inglehorough above the Level of the Sea, 431 



was supported by a firm pile of stones surmounted by a level 

 flag, but in calm weather it was occasionally placed upon a 

 large circular board, slightly concave at the upper surface, 

 which was screwed firmly by its brass centre to the staff of 

 the theodolite set up in the secure manner already de- 

 cribed. 



As the atmospherical refraction is generally fluctuating, the 

 best method of obtaining its mean value correctly from the 

 reciprocal observations and depressions, will be to class the 

 observations at the several stations according to their dates 

 rather than by the mean of the whole series. The following 

 list, constructed on this plan from sixty- four observations made 

 on eighteen different days in 1829, 1830 and 1832, exhibits a 

 mean refraction of about ^ of the contained arc. From 

 this ratio the deviation of any day's observations (which is 

 given in the last column,) is generally not only inconsiderable, 

 but occurs in small arcs, where its effect on the differences of 

 level becomes unimportant. With regard to the few striking 

 exceptions, the one at Farleton Knot, marked K, was derived 

 from a hasty measurement, on one arc only, of the elevation 

 of Ingleborough as it was rapidly becoming obscured by mist. 

 Those at Hest Wall originate, no doubt, in that horary varia- 

 tion of the refraction occurring in low situations on calm 

 bright days, which rendered so uncertam the measurement 

 of the height of the tide. All the observations at Clougha, 

 marked C, with the exception of the three first made — those 

 of Ingleborough,— concur irt giving a refraction in excess, 

 which may, perhaps, be accounted for by the wind, which was 

 scarcely sensible at first, soon after blowing a strong gale* 

 from the north-west, the quarter fronting the steep and lofty 

 acclivity on which the pikes stand, and occasioning a con- 

 densation of that portion of air through which the rays from 

 the several signals would finally pass. All the angles, but in 

 particular the concluding one, that of the Breakwater pole, 

 were uncertain to a few seconds, from the tremulous motion 

 of the pile supporting the sector, which had been constructed, 

 very improperly, partly of sods. The letters in the list refer 

 to the' dates given in the register of the observations. 



* The strong breeze off the bay of Morecambe, which had blown at 

 Hest Bank all the morning, subsided about the time the gale sprung up at 

 Clougha. 



