Major-General Roy’s Account of the 
and that the telefcopic Tpirit level gave a defcent of 36.1 feet 
from the lower mod; pipe to the mrface of dimmer water in the 
Thames at Hampton. The accurate lection of the river lately 
published, gives a fall of 13.33 feet from Hampton to the level 
of low water fpring tides at lfle worth. Now thefe three being 
added together, we have nearly fifty-three feet for the height 
of the bale above Iflewort-h. Having had no immediate means 
of determining what real difference there may be between Ifie- 
worth and low water fpring tides at the mouth of the Thames 
(for in fiance at the Hope or the More), I have fuppofed that 
fall to be about feveii feet, fo as to make the total defcent lixty 
feet. Now, fuppofing the fpring tides at the Note to rife eigh- 
teen feet, if, according to M. de la Lance’s method, we de- 
duel one-third of eighteen, viz, fix feet from lixty, welhall have 
fifty- four feet, or nine fathoms, that the mean furface of the fea is 
below the meafured bafe. Whether this conclufion be perfedlly 
accurate or not is of no moment, lince a wdiole fathom of 
difference (and I apprehend we are not farther from the truth) 
does not vary the reduction quite one-tenth 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 mod accurate meafurement 
imaginable is dill more liable to err in excefs than in defied, we 
will throw away fome ufelefs decimals, and edablilh the ulti- 
mate length of the bafe at 27404 feet and feven-tenths. 
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General 
