TRIGONOMETRICAL SURVEY OF INDIA. 207 



small alteration may be given to it longitudinally. This will be seen by 

 a reference to the apparatus where the plate being fixed to the box by 

 two brass screws, the long steel screws I have above mentioned, act in the 

 same manner as a tangent screw on the limb of an instrument, and draw 

 the whole bodily forward or thrust it backwards in a direction parallel to 

 itself. There is also to each camel a large milled headed brass screw, 

 which gives a differential motion laterally to each end of the bar. 



The registers which mark the limits of each set of bars in succession 

 consist of a frame of cast iron, of a pyramidal shape, surmounted by a 

 brass plate with a circular aperture of two inches in diameter. Through 

 this aperture a tube is adapted to slide up and down, at the top of which 

 is a circular head hollow, and perforated in the middle, so that an internal 

 plate on which four screws act at right angles to each other may be moved 

 differentially in a lateral or longitudinal direction ; this moveable plate 

 carries a very fine dot engraved on silver : some of these registers have the 

 tube made to slide below the frame altogether, in which case the ground 

 must be excavated from beneath it. Some are a mere triangular slab of 

 cast iron with a moveable brass register in the middle, they are used at 

 leaving off at night, and in cases where they are likely to be left standing 

 for any time, being less liable to derangement than the more lofty ones. 

 Each register rests upon a slab of stone sunk into the ground for the 

 purpose. 



The meaning of the sliding tubes is that the small dot on the move- 

 able plate may be kept as nearly at the same distance from the telescopic 

 micrometer as practicable, but in cases of ground which slopes greatly and 

 always where this is not attainable, a provision is made t'oi- ahcrini;- the; 

 focal length of this microscope l)y changing the object glass. A small 

 box which accompanies contains twelve sets of object glasses for tiiis 

 particular purpose, varying fioui tour to twelve inches, and there are four 

 others which are adapted to a diHorence of height so great as twenty feet. 



