80 General Walker — Error in Spirit -levelling operations. [April, 



tions as regards the general configuration of the ground levelled over as 

 appeared to be most reasonable and probable ; and he has arrived at the 

 conclusion that the levelling operations cannot have been influenced by 

 the proximate and local attractions experienced on the line of operation to 

 anything like the amount of the discrepancy met with. 



Thus the discrepancy is possibly due chiefly to error in the levelling 

 operations. As the most probable locus of such error was believed to lie 

 in the section over the Ghats, where the ascents were very steep and the 

 staves were sometimes so close to the levelling instrument that the foot- 

 marks could not be seen in the field of the telescope, that section was 

 re-levelled ; but the results of the two measurements were identical. And 

 indeed it seems highly improbable that the discrepancy can be due to any 

 accidental gross errors, seeing the special precautions which have been 

 taken — by the employment of independent operators and instruments, and 

 the use of double-faced staves — to guard against such errors. 



But it has long been known that all spirit-levelling operations are 

 liable to an accumulation of small errors which, though individually so 

 minute as to be barely appreciable at any single station where the instru- 

 ment is set up, have a tendency to be repeated at successive stations, and 

 may therefore attain a considerable magnitude at the end of a long line of 

 levels. In the operations of this Survey it is customary to guard, as much 

 as possible, against such errors by various expedients, — such as observing 

 the back stafl" first at one station and the forward staff first at the next, 

 alternating the direction of operation on succes-ive daj^s, or at least 

 executing half the work of a field season in the direction of the terminus 

 and the other half in that of the origin ; invariably setting up the staves 

 at equal distances from the instrument at every station ; and tilting the 

 instrument occasionally, to guard against the heating influence of the sun 

 — or the cooling influence of winds — acting on one side more than another, 

 and causing dislevelments which would be frequently repeated if not coun- 

 teracted, and thus create an accumulation of error. There is, however, a 

 liability to personal misapprehension in reading the bubble of the spirit- 

 level which may tend to produce a considerable accumulation of error on 

 lines of which the general direction is either towards the sun or opposite 

 to the sun. Owing to the level being placed above the telescope, the 

 observer gets a side view of the bubble, refracted obliquely through the 

 thickness of the glass tube, which is not so sharply defined as the look- 

 down view from above. The rim round the bubble, caused by the adhesion 

 of the liquid to the sides of the tube, becomes so prominent that its 

 extremities may be observed instead of those of the bubble. When light 

 falls obliquely and not vertically on the instrument, and either end of the 

 telescope is pointed towards the light, the outer-edge of the rim at the end 



