47'8 'Major-General Roy's Account of the 



and that the telefcoplc fpirit level gave a defcent of 36.1 feet 

 from the lowerrnoft pipe to the furface of fummer water in the 

 Thames at Hampton. The accurate fe6lion of the river lately 

 publiihed, gives a fall of 3 3._^3 feet from Hampton to the level 

 of low water fpring tides at Ifle worth. Now thefe thr^e being 

 added together, we have nearly fifty-three feet for the height 

 of the bafe above Ifleworth. Having had no immediate means 

 of determining what real difference there may be between Ifle- 

 worth and low water fpritig tides at the mouth of the Thames 

 (for infrance at the Hope or the Nore), I have fuppofed that 

 fall to be about feven feet, fo as to make the total defcent fixty 

 feet. Now, fuppofing the fpring tides at the Nore to rife eigh^ 

 teen feet, if, according to M. de la Lande's method, we de- 

 du£l one-third of eighteen, viz. fix feet from fixty, w^e fliall have 

 fifty- four feet, or nine fathoms, that the mean furface of the fea is 

 below the meafured bafe. Whether this conclufion be perfedly 

 accurate or not is of no moment, fince a whole fathom of 

 difference (and I apprehend we are not farther from the truth) 

 does not vary the reduclion quite one-teuth of an inch. The 

 reduced bafe has therefore been found by the following ana- 

 logy : as the mean femi-diameter of the earth (fuppofed here 

 to be 3492915 fathoms) augmented by nine fathoms, is to the 

 mean femi-diameter, fo is the meafured bafe 27404.7925 to the 

 reduced bafe 27404.7219 at the level of the fea. It will 

 doubtlefs be allowed, that infinite pains have been taken in the 

 field and otherwife, throughout the whole of this operation, to 

 obtain a juft conclufion ; but as the mofi: accurate meafurement 

 imaginable is flill more liable to err in excefs than in defeat, we 

 will throw away fome ufelefs decimals, and effablifh the ulti- 

 Miate length of the bafe at 27404 feet and feven-tenths. 



General 



