g94 SURVEY OF THB COAST 



The object end has a lengthening tube giving to this part 

 some preponderance, by which the eye end is pressed up- 

 wards against an arrangement of three small sliding tubes x, 

 which reach downwards from the limb of the circle, and 

 present the rounded end of the finger screw z' to the upper 

 part of the telescope. By the sliding of these tubes, and the 

 filial adjustment by the screw z'. and the micrometer arrange- 

 ment, the accurate pointing upon an object for the sake of 

 verification is obtained. 



As it cannot be my object liere to go into such details of 

 description as must be considered generally known, the above 

 is, I believe, sufficient to explain all the peculiaiities of the 

 instruments, and to detail their principles and use. 



Methods of Observing with the Two-feet Theodolite. 



The limits assigned to the present papers rendered it ne- 

 cessary, in the description of this instrument, to suppose that 

 a general idea of it might be obtained from the figures there 

 given. The same reason now compels me to consider the 

 general principles and method of levelling it, together with 

 the adjustment of its line of collimation and axis, as well 

 known, and to confine myself to the explanation of its pe- 

 culiar properties, and of some theoretical principles and prac- 

 tical advantages not hitherto treated of, at least to my know- 

 ledge. 



An observer furnished vvith an instrument with which he 

 has never oliserved should first ascertain its properties 

 and defects from the mathematical principles on which it is 

 constructed. This will be more indispensalile, when the in- 

 strument has had to undergo transportation. 



In this inquiry he will be much assisted, — if, besides his 



