OP THE UNITED STATES. 815 



An attentive observer will find the object pointed at al- 

 ways disappearinsj; when very near the wire, and as if it were 

 brok( n otl". In like manner, in pointinj;; with the ciossing 

 wires upon a signal prescntinj; a point, it will be impossible 

 to place it actually in tbe vertex of the angle ot" intersection, 

 and it will become visible only at a certain distance, stand- 

 ina; free between the wires, without admitting actual contact 

 with the wires themselves. 



Mditions made to the liepeciting Circle with tico Telescopes. 



The general principles and construction of this instrument 

 are well known from the descriptions given l)y French wri- 

 ters on the subject. The peculiar construction adopted Uy 

 Mr. Troushton is described in the English Kncyclopedias. I 

 sliall tlierefore suppose such constructi in known, and de- 

 scribe only the [)cculiarities of the two circles which Mr. 

 Trougliton made for me. 



In the usual constiuction of this instrument, when an ob- 

 servation is to be renewed for the sake of repetition, the front 

 telescope bearing tlie verniers is to be moved; no trace of 

 the foregoing observations is left ; their value in some mea- 

 sure is concealed l)y the position of the back telesco|)C or 

 level. In celestial observations clouds or intervening cir- 

 cumstances in general, which occur so frequently in o!)ser- 

 vations, may at this moment render all previous attention 

 and care useless ; and the observer feels always somewhat 

 anxious on that accoimt. 



It is however evidrnt that the successive steps of the level 

 or back telescope measure the an2;lf" in tlie same manner as 

 those of the front telescope. V\)nn this consideration I 

 founded a construction, liy which the instrument 2;ives two 

 separate scries of angles upon the same division, from the 



