6 HYDROGRAPHICAL SURVEYING [chap. i. 



Heather's This useful work, which should be in the hands of every sur- 



instru-"^ veyor, goes so fully into the construction of instruments, and 



ments. in most cases into the methods of ascertaining and, as far as 



may be, correcting their errors, that we shall refer the reader 



to it on most points, adding only certain practical suggestions 



that are not therein mentioned. 



HADLEY'S SEXTANT. 



It is not, perhaps, necessary to say much about the sextant, 

 as so many works have already treated the subject ; but there 

 are several practical points not generally mentioned, which 

 may be of value in selecting a sextant with a view to the work 

 of a nautical surveyor. 



Besides those noted by Heather, then, — 



1, One of the eye-pieces of the inverting telescope should 

 have a high magnifying power, about 15 diameters, as contacts 

 of the sun's limbs in observations mth the artificial horizon are 

 far easier made the larger the suns. 

 Daxk Eye- 2. Several dark eye-pieces should be provided, with neutral 

 tint glass in them of different intensities. These should be 

 fitted, not to screw on to the eye-piece, but ground conical, to 

 slip on to a similar conically ground surface on the telescope 

 eye-piece. These %\ill be found very useful on cloudy days, as 

 a httle practice will soon enable the observer to substitute one 

 shade for another in a fraction of a second, as clouds sweep on 

 or off the sun, and many sights will thereby be saved. It is 

 very important to have the suns in artificial horizon observa- 

 tions, of the same brilliancy, and for this reason the hinged 

 shades on the sextant should never be used for the purpose ; 

 as, in the first place, they introduce error, and also, if the shades 

 have to be altered to suit the varying brightness of the sun 

 during the observation, the suns will be of different brilliancies, 

 as these shades are never of the same tints. 



By using the dark eye-pieces, the up-and-down piece,* when 

 adjusted to equalise the suns, will bring the axis of the telescope 

 nearly exactly in hne with the edge of the silvered surface of 

 the horizon glass, which is the best position for observing, and 



* The up-and-down piece of a sextant is the portion that bears the collar 

 for the telescope. 



