TRIGONOMETRICAL SURVEY OF INDIA. 



205 



The second bar is now brought up to the first by placing the rear dot 

 under the advanced microscope attached to the first bar, and raising or 

 lowering it to focal distance. In the same manner the .3rd, 4th, 5th and 

 (Jth, when the series is terminated by fixing one of the triangular registers. 

 After the length of the whole six bars has thus been taken off, the leading 

 register is left standing and another set of bars is placed and measured 

 off in the same manner. 



For the better preservation of an equable temperature, the whole of the 

 measurement with these bars is carried on under shelter ; a certain number 

 of tents of a suitable form being constructed on purpose, and especial care 

 is taken to preserve the apparatus from the direct action of the sun's rays. 



The adjustments of the sets of microscopes consist in placing the 

 side telescope parallel to the plane which passes through the lines of 

 collimation of the two microscopes, this is done as follows ; direct by 

 means of its sights, the six inch brass ruler to a mark on a piece of paper 

 fixed at thirty or forty feet distance on a wall, and place over it the two 

 microscopes witli each of the dots under a wire, then direct the side teles- 

 cope to the same paper, and mark the point it cuts. Now turn the set of 

 microscopes half round in azimuth, so that the positions may be over the 

 contrary dots to what they were before, and having turned the side teles- 

 cope half over in altitude, mark the point on the wall which its Avire cuts 

 in this new position. If then the distance between each of these points, 

 and the middle point be exactly ecjual, the line of collimation is quite 

 parallel to the line running through the microscopes, otherwise it must be 

 adjusted half by means of the screws, wliich affect the axis, and hall' by 

 those which alfect the line of c olliiualion. 



The measurement of every individual sot of bars is taken horizontally, 

 but it docs not follow that llu- sni lacr ovt-r wliicli it is ( (iiiducii'd need be 

 horizontal; for each trcssel stands upon iliicc i)Kkcls driven miu ihc 



