1864.] Report of the Great Trigonometrical Survey. 389 



trial line has been carried over a distance of eight to ten miles, the 

 ground beyond is carefully reconnoitered for a suitable site, to which 

 a line is cut from a convenient point in the trial line ; thus two sides 

 and the included angle of a triangle are given, with which data it is 

 easy to ascertain the direct line between the two stations, which is 

 then cleared to obtain mutual visibility. Owing, however, to the 

 valuable nature of the property through which the triangles were 

 carried, it was necessary to run a traverse along each line, with numer- 

 ous intermediate bends, to avoid houses and orchards. In clearing 

 the final line, great caution was requisite to prevent any tree from 

 being cut down needlessly, a matter of some importance in Bengal, 

 where every tree is more or less valuable, and has to be paid for. 

 These circumstances greatly increased the labour of the preliminary 

 operations, and protracted them over a longer period than is usual. 



Further delay was caused in building the principal stations. These 

 are usually, towers, with a central pillar, four feet in diameter, of 

 burnt brick and lime masonry, surrounded by a platform of unburnt 

 bricks and mud, fourteen to sixteen feet square, the whole raised to a 

 height of twenty to forty feet, according to the nature of the obstacles 

 to be overlooked. This structure has been adopted on account of its 

 cheapness, and the rapidity with which it can be constructed ; it has 

 hitherto been found to be well adapted for our requirements. But it 

 appears to be inapplicable for the rainy and moist climate of Eastern 

 Bengal, where unburnt bricks rarely have an opportunity of drying 

 sufficiently to be safely used, in raising a structure of such necessarily 

 large dimensions. At one of Lieutenant Thuillier's stations, in conse- 

 quence of the employment of damp materials in the unburnt brick 

 work, and constant and heavy falls of rain during the construction, 

 the building gave way, under the weight of the instruments and 

 observatory tent. Fortunately, the large Theodolite was packed in 

 its case, and received no injury, but the season was too far advanced 

 for the tower to be rebuilt before the setting in of the monsoon, and 

 as the mishap occurred in the first polygon of the principal triangula- 

 tion, and there were no more towers ready in advance, the outturn of 

 work, as measured by the area triangulated, is unusually small, though 

 much valuable experience has been gained, and there is every reason 

 to hope that there will be a full out-turn of work next season. The 

 design of the tower stations will have to be altered to suit the climate 



