396 Mr, atwood’s 'theory for the Menfuration 
diftance between that point and any other celeftial objeCt in the 
plane of the feCtor, This method, however, is evidently 
imprafficable, unlefs the inftrument can be fteadily fixed ; for 
which reafon aftronomical quadrants become ufelefs at fea ; and 
from the difficulties which attend placing them in their due pofi- 
tion and adjuftment on firm ground, they are almoft wholly 
confined to regular obfervatories. 
Mr. hadley*, by an ingenious application of optical prin- 
ciples, contrived to bring both extremities of the arc meafured 
into the field of the fpeaators’s view at the fame time ; by 
which improvement, angles are taken at fea, as well as on land 
with an unfixed inftrument, to a degree of accuracy fufficient 
for nautical and other purpofes, when the utmoft exaCtnefs is 
not required. 
Mr. hadley’s invention is a particular cafe of a very 
extenfive theory, as yet but little attended to. According to 
his method, which is well known, the two reflecting furfaces 
ufed in the obfervation are perpendicular to the plane of mo- 
tion; the direction of the telefcope, and of the rays paffing 
between the reflectors being parallel to that plane ; whereas the 
inclination of the telefcope, and of the intermediate rays, as 
well as of the reflectors themfelves to the plane of motion, 
admits of unlimited variety. A general theory to determine 
the angle obferved by two reflections from the data on which 
its magnitude depends, without limitation or reftriCtion, feems 
applicable to feveral ufeful purpofes in practical aft ronomy. Hav- 
ing never feen any geometrical cbnft ruCtion or analyfis of this 
curious problem, I was induced to beftow fome confideration 
* Phil. Tranf. N° 420. See alfo a trad, intituled, The Theory of hadley’s 
^U&drant, by the rev. w. ludlam. 
Oil 
