248 Mr. Nixon on the Trigonometrical Height 



stance of an ox*s eye the indices are as nearly as possible in 

 the required ratio. 



3rdly. It follows as a necessary and mathematical conse- 

 quence that an eye may be perfectly achromatic for direct 

 centrical pencils, though not for excentrical and oblique pen- 

 cils, which appears also to accord with observation. 



These positions, I believe, will be found to stand quite in- 

 dependently of Sir D. Brewster's remarks, which may yet be 

 perfectly correct in reference to the particular cases examined, 

 and I sliould be among the first to acknowledge their value 

 as bearing those marks of excellence which characterize all 

 the observations of their author. * 



Oxford, March 3, 1835. 



XLII. Trigonometrical Height of Ingleborough above the Level 

 of the Sea. Part I. Bi/ John Nixon, ^sg.* 



[With a Plate.] 

 l\yTY standard altitude, that of Ingleborough, having been 

 -^'-■- originally determined from insufficient dataf, the details 

 of a more satisfactory measurement are now presented. 



All the requisite observations might have been made at any 

 of the numerous places on the margin of the sea in the bay of 

 Morecambe, which command a view of Ingleborough and 

 several other stations of the Ordnance Survey; but as dif- 

 ferences of level may be measured with })eculiar advantages 

 from intermediate positions, four eminences situated between 

 Ingleborough and the bay were fixed upon as stations whence 

 the elevation of the hill and the depression of Hest breakwater 

 pole could be observed. 



A brief description of the stations and their signals may not 

 be deemed superfluous. About two miles north-east of Bur- 

 ton-in-Kendal a saddle-shaped ridge, chiefly of limestone, ex- 

 tends upwards of two miles from north-west to south-east. 

 The precipitous northern end, noted for its resemblance to 

 the Rock of Gibraltar, is called Farleton Knot, Near the 

 south-west side of the boundary wall that runs along the 

 watershed of the ridge, a lofty pile of huge fragments of rock, 

 constructed, from the difficulties of the situation, of an irregu- 

 lar figure, was erected upon a highly inclined bed of lime- 

 stone, close to its basset e{\gQ at the loftiest point of the Knot. 

 The southern, more gradual termination of the ridge, is 

 known by the names of Hutton-Roof Moor (or Crags) north- 

 east of the division wall, and of Dalton Fell south-west of it. 



* Communicated by the Author. 



f See Philosophical Magazine for 1823, vol. 1x5. p. 262. 



