OP THE UNITED STATES. 3-15 



ment may be varied in clifTcrcnt inaiiiiers, without altering 

 its principal qualities. I made various plans ; but it would 

 be needless to state here their varieties. 



Alethnd of Ohxfin'ing imth the Repeating Reflecting Circle. 



The adjustments of this instrument being of course in eve- 

 ry respect the same as tlie well ivnown adjustments of any 

 reflecting instrument, must not be repeated here. In all re- 

 peating instruments, attention is required, to avoid mistak- 

 ing in tlie regular course of alternating observations, and use 

 of clamps and screws. It is therefore necessary to proceed 

 at first with measured and cautious steps, and to form a re- 

 gular habit of an order easy in the manipulation, wiiich, when 

 it becomes habitual, will always proceed more surely and 

 rapidly. The examples of observations given in the exem- 

 plar of the Day-Book and Journal will prove that two series 

 often altitudes may be taken in the space of five or six mi- 

 nutes. 



The correspondence of observations with this instrument 

 and others with the eighteen inch repeating circle, whicli I 

 had an opportunity of making at the northern boundary line, 

 proved tiiat the former was capable of giving an accuracy 

 nearly as great as the latter. 



The most convenient order of proceeding in a repeated 

 double series of observations is the Ibllowing : — 



i. Place the vernier A upon 0° or any desired or roimd 

 number ; and read off B. 



2. Write these readings in the third and fourth columns 

 of the Day-Book. (Vide F.xcmplar.) 



3. The alhidade DC i)cing undamped, make an observa- 

 tion, by the motion of the mirror and circle together, the con- 



