334 SUHVEY OF THE COAST 



will move the front telescope near the other stoj) e, and make 

 the second observation, pointing by the tangent screw of this 

 telescope on the arm D, while the assistant will adjust the 

 level, if it should be necessary, by the circle screw at ni; 

 though this adjustment will be very slight, if any, wiien ihe 

 circle is well adjusted. 



The time of the simultaneous coincidence of the pointing 

 and adjustment of the levels is again noted ijy the secretary, 

 and written in the first column before tlie readings W, ^c. 

 constituting the first time of the series of angles witii the 

 back alhidade. 



Before turning the circle off, the observer will take care 

 to place the other horizontal stop n to the contact with tlie 

 alhidade A from this second position. 



From this, the circle will be placed again in the first posi- 

 tion, and the observations continued regularly and in the same 

 order exactly to the last of the series, which must be like the 

 first, an observation in which the level is placed by its own 

 screw at /*, so that the whole series consists of an odd number 

 of observations. 



The series being thus closed, all verniers of i)oth sets are 

 read off, those of tiie front telescope are written opposite to 

 the time before the last, and those of the back alhidade to 

 the last time, prefixing again in the second column the first 

 letters of the set of readings. 



All these will be observed in their regular order in the 

 corresponding examples of the Day-Book. 



By one observation more than is usual in the other circles, 

 these give therefore two complete, equal, and (so far as le- 

 fers to readings) independent series of observations. To 

 that of the fiont telescojie belong all the times, the last ex- 

 cepted, and to that of the back alhidade all the times except 

 the first. They form a check upon eacii other against mis- 

 takes or errors, and may in some measure serve as a test of 

 the proportional accuracy of different series of observations, 

 besides that all results are evidently doubled. 



The convenience obtained byjhe use of the screw m. for 



