the Trigonometrical Operation . 237 
ground and Caftle above the faid gun) to be 465.8 * feet above 
low water at fpring tides. Having alfo meafured a bafe for the 
purpofe, he determined the height of the cliff geometrically, 
which agreed within lefs than a foot of the refult by levelling. 
In juftice to this very meritorious Officer it is incumbent on me 
to lay, that not only on this occafion, but on every other during 
the progrefs of our operations near Pover, his affiftance was 
moft eflentially ufeful. 
The height of the ball of St. Paul’s above the Thames at 
Paul’s Wharf, and the height of Shooter’s Hill Inn above the 
Gun Wharf in Woolwich Warren, were feverally determined 
in 1773, at which time the experiments were carrying on for 
the purpofe of finding a theory for the meafurement of alti- 
tudes by the barometer. 
The height of Severndroog Caftle, lately built on Shooter’s 
Hill, has fince been deduced from that of the Inn. 
Lafrly, the altitudes of all the intermediate ftations, as 
exprefled in the three colums towards the right hand of the 
general table of refults, placed at the end of the preceding 
feftion, have been eftablifhed by the reciprocal angles of eleva- 
tion or depreffion, gradually carried on from ftation to ftation, 
throughout the whole feries of triangles, whereby the two 
extremities are connefted together ; and no greater uncertainty 
has been found at Hampton Poor-houfe than a few feet, occa- 
fioned no doubt by the uncertainty of terreftrial refradlion : for it 
is to be remarked, that, to the weftward of Greenwich, no double 
but only fingle obfervations were obtained; wherefore, the 
* Sir Thomas Hyde Page, when engineer at Dover, at my requeft, had been 
fo obliging as to order his workmen (he himfelf being ill at the time) to deter- 
mine the height of the turret of the Keep, which, by a miftake of about nine 
£eet in the height of the cliff, they made 475 a ^ ove low-water mark. 
rela- 
