12 



HYDROGRAPHICAL SURVEYING 



[chap. I. 



Amalga- 

 mated 

 Mercxxry. 



Horizon 

 Glass 



fully round with a sharp knife, and varnish lightly over, either 

 ■\\Hth the clear stuff used by the instrument makers, or with 

 varnish that can be made on board, by dissolving sealing-wax 

 in spirits of wine. 



The glasses of some sextants seem fitted on purpose to invite 

 the damp to penetrate between glass and silvered surface. 

 These will want protection by sticking thin strips of paper 

 along the edges exposed, and well varnishing. In some cases 

 a stopping of thick amalgam, placed between the glass and 

 the frame at the back, where there is one, will answer well, 

 and prevent any damp getting at the back of the glass at all. 



The mercury which remains Avill contain tinfoil in amalgam, 

 and should be preserved in a bottle by itself, draining off the 

 thick of the amalgam by a sharply twisted paper funnel. It 

 can then be used again for resilvering. Care must be taken 

 not to allow any of this to get into the artificial horizon bottles, 

 as the smallest quantity of it will spoil a whole bottle of pure 

 mercury, and the amalgam can only be removed by evaporation. 



Notwithstanding, mercury containing tin in amalgam can be 

 used for artificial horizon work, by carefully sweeping the sur- 

 face after it is poured out, with a piece of paper. Some ob- 

 servers have gone so far as to prefer amalgamated mercury for 

 this purpose, but we do not agree, except when used in connec- 

 tion with the amalgamated trough described on p. 15. 



In resilvering an horizon glass, only the portion required 

 should be operated on, leaving one half clear. The edge of 

 the foil must be sharply and smoothly cut before applying 

 the mercurj^, and not the smallest nick or cut permitted to 

 remain in it. 



SEXTANT STAND. 



Though a practised observer will get good observations in 

 an artificial horizon, with a sextant without a stand, he will 

 get them far better with one, and in all work where accuracy 

 is aimed at, a stand should be used. 



Unsteadiness of hand, to which all are so liable, from previous 

 exertion, indisposition, and many other causes, is put out of 

 the question by using a stand. 



With star observations this is especially the case, as it is 



