on Rivers and Canals. 649 
obfervations is highly ufeful, and among mountains 
(where mine can be of little or no fervice) far preferable 
to every other hitherto difcovered ; yet it will eafily be 
acknowledged by every one who is acquainted with what 
M. de luc fmJ , Sir George shuckburgh, and Colonel 
ROY have done upon this fubjedt, that the greateft at- 
tention to an infinity of varying circumftances, as well 
as the greateft nicety and exadtnefs both in the inftru- 
ments and in repeated obfervations, are necefiary if we 
would come at the truth thereby. 
Again, the method of taking the difference of heights 
by the quantity of declivity in rivers requires no atten- 
tion to the curvature of the globe, an objedt (as every 
one knows) infinitely too confiderable to be negledled in 
the common method of levelling; as are alfo the great 
and varying refradtions of the vifual rays fo near to the 
furface of the earth as they muft be taken in the pradtice 
of that method. The quantity of efFedts and of errors 
in the vifuals proceeding from this laft caufe muft be 
very different at different times, as it depends wholly on 
the greater or lefs denfity, on the greater or lefs quantity 
of vapours fufpended in the loweft part of the atmo- 
(m) See his work on Barometers and Thermometers, in two vol. quarto; 
(n } See the learned and curious treatifes of thefe two gentlemen in the 
Fhilofophical Tranfa&ions for 1777. 
fpherey 
