1830.] 
of Geodesy. 
5 
17. The most obvious method of applying this principle to practice, is to mark 
out, or trace on the ground, two lines forming any angle, (a right one would be the 
most convenient,) and then proceeding along these lines with an instrument for letting 
fall perpendiculars 10 measure, as we go along the distances to the points at which 
perpendiculars to the lines from the several objects would fall. Then drawing these 
on a proper scale on paper, their intersections would give the positions of the seve- 
ral points. 
18. Such a method is excellent, as far as it goes 1 S but it requires that all the 
positions should be in sight, from the two lines forming the origin of the coordinates • 
if we wish, at least, to preserve the efficiency of the principle, the independence 
of the results. This condition of course seldom obtains, and we must therefore ha e 
despaired to apply the method on the great scale, did not practical Astronomy sup- 
ply the desideratum. ^ 
19. It is evident, that if we consider the Equator and one of the meridians to repre- 
sent the axes of the coordinates, then will the latitude of a place be one of them, and 
the distance from the meridian (which is only another expression for the longitude) 
the other. By determining then our latitude and longitude, we afford data for draw- 
ing the coordinates, the intersection of which will fix the place of any point. 
20. The facility of the method is great, at least in the case of the latitude. The 
altitude of a star being observed, gives us this element ; and the labour of observation 
and calculation is equally trifling. But with regard to the accuracy of the method 
there is a limit necessarily placed to it, in the smallness of our- instruments, which 
tor the time, are made to represent the whole meridian or circuit of the dobe. The 
scale is, therefore so much smaller than that of any map, that very trifling errors be- 
come magnified into such as are scarcely admissible in every map. With regard to the 
amount of this error it will depend upon the instruments used; aTroughtou’s reflec- 
tive circle is one of the most ordinarily employed, but the accuracy attainable with 
tIT T t m > tn u n ? n t lS> P , Crh * ps ’ SCiirCt>, .V sufficient, except for very general maps. A 
Irou 0 liton s altitude and azimuth circle of 15 or 18 inches, furnished with three or 
01 ' even seven microscopes 1 - would be doubtless capable of giving much more 
c di rect lesults; but though portable, such an instrument is troublesome to transport 
and consumes much care and time in the use : it is also very expensive* The former 
might be depended on, perhaps, to 20 in a single observation, the latter to 10 " 8 " 
or 6, with one reversal, according to the number of microscopes used : a mean of 
many observations would, probably, be correct to half these quantities. But the 
preferable instrument to either, is the repeating and reflecting circle. This instru- 
ment, which was invented by Borda, has never been much patronised by English 
makers, or English observers, owing, perhaps, to the prejudice in favour of Trough - 
ton s circle. Some years ago I ordered one from Dollond,and though circumstances 
picyen e my gmng the instrument a fair trial at the time, and occasioned me im- 
mediately afterwards to part with it, I saw enough of it to be convinced that it was 
»r ± DdS °f P aMe ol f, erver ’ of Kiv-ing, perhaps, as accurale results as 
could be had with much larger and less portable instruments. The method of re- 
tiucmg any ot the heavenly bodies to the meridian when near it, so well explained 
and illustrated by Delambre, enables us to multiply the observations to any desira- 
ble extent, so that in a few hours a result may be obtained with this instrument, of 
angle of 90°^ JtCtU1 ® C1 ° SS ’ aS * S ca * ,ed > or still better, the pocket sextant, set to an 
thino* p,an ® xt , ent > 1 ca . nn imagine a better method, unless it be a some- 
an?n,rm:,! ar l ne L W, : 1Ch ?'“ deed pnndple the same. Measure three distances, or more, 
t u P s, » na,s at each point. Then, with a compass, take the bearings of as many of 
beSn T u T' be coav ? UIcnt .v had at each position you wish to lay down Three 
bearings would be a sufficient cheek on each secondary position, and* if the nrinnrv 
on > 10I f mr hXe - tr !f on °metrical proceedings, this method would be a very correct 
one of filling- m detail, the principle of the independence of the resulS bein- 
full) apparent. A small pocket sextant would be still better than the compass as by 
didesthe mdifl? S £. bt , euded b y.. thr , ee being taken, the intersections ’of two 
a"dn a nkJ whl ? h an ; eas,I y f 1 ound ’ ™ld give the place of observation. If 
that Van b ? SU )St,tu ] ed ’ A we 1 should bave the very best method of land survey- 
AlUl ye,) Sto “ ge tOS ^ il >“ »"» «i<dr 
ti!„ 2 - Pe I h ? p8 so lna,, y as seven are usp d, Verniers would answer the purpose 
The inventor of this instrument has, lam aware, given it as his opinion, (Trans 
5'* 8o S* >0 ‘ !•’ p ; | hat more tbau tvvo microscopes cannot conveniently be at- 
^ ,' ed tc | * ie vert,c al circle. Some little awkwardnes may certainly be occasioned in 
admg them, hut not sufficient to authorize our rejecting so capital an improvement. 
