the Trigonometrical Operation . 
245 
fore, Thefe refults are not however offered as being free from 
error; on the contrary, if the circumftances had permitted this 
to become a principal objedt in our operation, the fucceffive 
repetition of the obfervations for many times would, no doubt, 
have furniftied ftill more fatisfadtory conclufions. It is hoped, 
neverthelefs, that thefe, fuch as they are, may have their ufe, 
were it only by fhewing the variablenefs of terreftrial refrac- 
tion, to induce to the making of others, which, as has been 
already obferved, would ultimately lead to a much more mi- 
nute inveftigation of this curious and interefting fubjedt. 
The heights of the barometer and thermometer are inferted 
on the days on which the obfervations were made, merely to 
fhew what was nearly the ftate of the atmofphere at the refpec- 
tive times. But we have not attempted to apply any corredlion 
on that account, becaufe it could not be done in a fatisfa&ory 
manner, and coniequently could not be ufeful, unlefs the cir- 
cumftances had permitted reciprocal obfervations to have been 
made at correfponding times with double fets of inftruments, 
which in our fituation was impoffible, 
By attending to the refults in the tahle, it will in general be 
feen, that terreftrial as well as celeftial refraction certainly di- 
ffiinifhes as the heights of the ftations above the fea increafe; 
and that, at particular times at leaft, it is much greater than 
has hitherto been fuppofed, even to between § and §■ part of 
the arc of diftance, inftead of being only gth or part. 
Befides the inftance of this extraordinary effedt inferted in the 
table, between Allington Knoll and Ruckinge, where the dif- 
tance of the ftations is but fmall, and one of them little 
high er than the fea, we could have given another on a diftance 
as well as on heights ftill more confiderable, namely, Shooter’s 
Hill and the ball of St. Paul’s Church : for, fuppofing the 
2 ' firft 
