314 SURVEY OP THE COAST 



3. In all other positions of tlie sun, with respect to the 

 two signals, the difference of the two corrections must be 

 subtracted fiom the angle. 



The angle between the sun and the signal determining the 

 angle at the centre of the signal between the station and the 

 reflecting point, and being bisected, it is evident that the ac- 

 curacy obtained by tiie method described is fully adequate. 



When the sun does not shine, and the state of the atmos- 

 phere is such as to affect no reflecting point, which will hap- 

 pen in dull and cluudy weather, the whole signals will ap- 

 pear like a while surlace and \n lull sl/.e ; and accordin2;ly 

 if its centre be observed, no reduction will be requisite. 

 The state in which the signals are seen is therefore one of 

 the ne(;essary remarks to be inserted in the journal. 



The system of wires in the telescope is to be adapted to 

 the form of the signals, in the same manner in wliich the 

 wire anangement in the microscope is adapted to the kind 

 of division of the limb. In this instrument the division be- 

 ing by lines, the wire arrangement in the microscope is that 

 of wires crossing each otiier under an angle of about thirty 

 degrees, which in placing it by the micrometer upon the di- 

 vision, will present this angle bisected, and enable us to judge 

 accurately of the coincidence. 



With a similar view, I suppose, Mr. Ramsden applied this 

 arrangement to the telescope of General Roy's theodolite in 

 the English survey, where staffs were used as signals, wliile 

 the division of the instrument being with points, the micro- 

 meter wires were simple perpendicular wires. 



For my signals therefoi'e, the perpendicular wires were 

 best adapted ; and as they were fine and exceedingly well 

 defined cobwebs, they showed the light of the reflected point 

 of the signal on both sides, by the irradiation, which of course 

 afforded a very nice pointing, far preferable to the contact 

 on the side of the wire, which has sometimes been substi- 

 tuted for the l)isection by the middle. 



I shall add one remark more : — 



