meafufing Heights with the Barometer. - 769 
the barometer fo much more complicate and difficult, 
as perhaps to deter others from applying it to ufeful 
purpofes. To this I arifwer, that though it feem utterly 
impoffible to render what is really intricate in its na- 
ture, extremely fxmple ; yet that the bell and furell 
method of arriving at limplicity at laft will, in the firft 
place, be to afcertain the limits of deviation of the 
rule, by a proper number of good obfervations, made 
in circumftances and fituations as different as poffible 
from each other. In the prefent flate of the matter, I 
doubt not but the barometer will be found to give re- 
fults fufficiently near the truth for all ordinary purpofes, 
the nicer bulinefs of levelling alone excepted. It is the 
only inftrument by which the relative heights of places, 
in very great and diftant trails of country, can eafily and 
fpeedily be obtained, by the preffure of the atmofphere 
alone. The method of tiling it is attainable by all, re- 
quiring only a little habit, and fome degree of attention 
to prevent the admiffion of air into the tube. Few peo- 
ple are qualified for the tedious and very laborious ope- 
rations of accurate geometrical meafurements. Moun- 
tainous countries rarely afford bafes of fufficient length, 
which, to avoid error, muff be meafured again and 
again with the utmoft care. Inftruments of the moil ex- 
penlive kinds mull be employed to take the angles ; at 
the fame time that a thorough knowledge of their ule, 
5 F 2 and 
