1863. | Progress of the Trigonometrical Survey. 115 
sistants of the Great Trigonometrical Survey, were consequently sent 
to revise them with the great theodolite, while Mr. Armstrong was 
selecting stations and building towers on the line of the Sutlej. 
Twenty-one principal triangles were ably and rapidly revised, after 
which Lieutenant Thuillier proceeded to join the Kashmir party 
while Lieutenant Herschel took in hand the Sutlej triangulation.* 
This consists of a series of single triangles, of which one flank rests 
on the sand hills fringing the Bahawulpore desert, and the other in 
the lowlands which are periodically inundated by the Sutlej. Thus 
the greater portion of the rays traverse moist jungles of tamarisk 
and long grass, -alternating with ridges of sand, forming a combina- 
tion which is peculiarly troublesome in disturbing the atmosphere, 
* Lieutenant Herschel took astronomical observations for the direct determin- 
ation of azimuth at 3 stations at an averave distance of 72 wiles apart. His 
mean triangular error is 0.53. In 85 angles his mean probability of error is 0.25 
between extremes of 0.10 and 0.38. He has given the following interesting table 
as a test of the accuracy of his work. 
f { 
(A) Maximum difference between observations. 
(B) 0” 1 | 977 | 97 | 4” BY | 6” 7 8” cs 
(Saebes of measures in aj to | to | to | to | to | to} to} to] to} & 
set. 1” Oe 3// 4!’ 5 Gr’ az 8” QO” 
2 EO OOP OM OO Ose Ol a4 
3 223 |251 3 (6) 0 0) (0) 0) O | 567 
4 810135) 65) 22 1 O Oh @ QO | 104 
5 (0) 5) 4} 29) 18 1 O 8) O 5Z 
6 0) 0 0 5] 11 5 3 0 O} 24 
vi 0 O 0) 0) 0) (0) 0) 0) 10) 0) 
8 0] O 8) 0 0) 0 0) 1 1 1 
9 (0) 0) 0) 0 1 0) O 0 0) 1 
Total. 227 1269 |162/) 56] 26 6 3 1 1 | 750 
Total 0” to 3’ 658. Total greater than 3” == 92. 
In this Table the unit is a set of measures of an angle on a single Zero, the 
arguments being A, the maximum difference between the respective measures 
forming a set, and B the number of measures. 
Lieutenant Herschel has introduced an improvement in the referring marks at 
present used in the survey. Instead of having two apertures one for a lamp, 
the other for a heliotrope, he made both lamp and heliotrope illuminate the same 
piece of ground glaas, the aperture of which was limited by a circular diaphragm 
of diameter suitable to the distance. Thus one object is intersected instead of 
two, and there is no flickering or unsteadiness of signal from wind or imperfect 
direction of heliotrope; there is no dazzle from too bright a sun, nor total dis- 
appearance in its absence, for the mere reflection of the sky suffices to illuminate 
the glass in tolerably clear weather. Ono mile is considered the best distance 
for such a mark. 
Q 
