1882.] General Walker — Urror in SiJirit-levelling operations. 81 



of the bubble towards the light is more olearlj^ defined than the inner, 

 while at the opposite end ot' the bubble the inner-edge of the rim is more 

 clearly defined than the outer. Consequently there is a tendency to assume 

 the instrument to be level when in reality the end towards the light is 

 depressed ; and though this tendency would probably vary in magnitude 

 with different observers, it is likely to affect all persons more or less. 

 Obviously it is uninfluenced by reversing the direction of operation, though 

 it disappears whenever the direction is at right angles to that of the light. 



This personal error in reading the spirit-level and setting the instru- 

 ment would obviously produce a maximum effect on lines of which the 

 general direction is meridional, when the operations are carried on equally 

 before and after noon ; when carried on between sunrise and midday, as is 

 more usual in this country, the direction of the line of maximum effect 

 would be south-east by north-west. The result would be apparently to 

 raise the southern stations relatively to the northern ones. Now this has 

 been found to occur in a greater or less degree in all our lines of levels 

 connecting tidal stations, and not only on the direct line from Bombay to 

 Madras, — to the following extent in each instance : — 



(1) From the mean sea level of the tidal station of Okha, at the 

 entrance to the Gulf of Cutch, to that of Bombay, there is an apparent 

 rise of 'SS feet, length of line 580 miles. 



(2) From the mean sea level of Bombay to that of Karwar, there is 

 an apparent rise of "93 feet ; length of line 530 miles. 



(3) From the mean sea level of Karwar to that of Madras, there is 

 an apparent rise of "92 feet ; length of line 560 miles. 



The apparent rise of 3 feet at Madras as compared with Bombay was 

 generated on a line of 730 miles in length ; it is thus materially greater in 

 proportion to the length of line traversed than the discrepancies met with 

 on the three other lines. But as all the discrepancies are in the same 

 direction, and all agree in raising the southern points relatively to the 

 northern, it appears possible that all the operations may have been in- 

 fluenced by oblique illumination of the bubbles of the spirit-levels. It is 

 to be noted that Madras is meridionally 400 miles to the south of Bombay, 

 and that an error of level adjustment amounting in arc to only l"-2, and 

 recurring with the same sign at only one-fourth of the stations at which 

 the instruments were set up — the operations being otherwise errorless — 

 would produce a discrepancy of the magnitude which has been met with. 



It has been considered desirable to enter very fully into the matter of 

 this discrepancy, because long lines of level so frequently close without any 

 appreciable error that disappointment may naturally be felt at the occur- 

 rence of so large a discrepancy as this, and the re-levelling of the entire 

 line may be considered desirable. But the weakest section has already been 



