1840.] Fort William, and Futtehghur. 89 



the back observation, for the altitude of the sun or a star 

 may be taken. 



To conclude, I can from long experience of its excellent 

 properties, very confidently recommend to my brother officers 

 of the Indian Army, the use of Troughton's reflecting circle, and 

 also of a small theodolite, which I will describe, as it was 

 lately constructed under the following circumstances. When 

 I was in England, my opinion was demanded at the East India 

 House as to the best construction of theodolites required for the 

 revenue surveys, in the North-west provinces. I well knew the 

 defects of the instruments hitherto supplied to the government, 

 which were unsteady and top-heavy. I accordingly consulted 

 Mr, Simms, and we agreed on the construction of the instru- 

 ment, I will now describe. 



This theodolite, though small, being only five inches in diame- 

 ter, is of a stout firm make, the azimuthal circle has three verni- 

 ers, and by it horizontal angles can be taken with much exact- 

 ness, by taking them on both arcs, in the same manner as in 

 the reflecting circle, each angle being from the result of six 

 readings. When more exactness is required, several observa- 

 tions should of course be taken, and as a further check, the an- 

 gles may be repeated on different parts of the limb, due atten- 

 tion being always paid to the lower, or watching telescope. The 

 vertical angles are taken on a complete circle, which being capa- 

 ble of reversion has many advantages as to correct observations 

 and means of adjustment. Altitudes and depressions are to be 

 taken with the face of the vertical circle in one direction, it is 

 then to be reversed in azimuth, and the operation repeated, there 

 I being room for the telescope to be turned over, as is done with 

 astronomical circular instruments, and the vertical angles repeat- 

 ed. There are two levels as usual, but the correction of the 

 line of collimation is best effected by taking the direct alti- 

 tude of a stationary object, and its reflected image by depression 

 on quicksilver. By this last mode of observing, also, a desirable 

 degree of approximation to the latitude may be had when re- 

 flecting instruments are not at hand, or cannot be used. 



A good sized magnetic compass is part of this theodolite, and 

 can be applied above it, when required. 



N 



