14 , HYDROGRAPHICAL SURVEYING [chap. i. 



the latter is correctly pointed to the centre of the artificial 

 horizon, 

 stools for Small three-legged stools about 14 inches high, on which to 

 stand. place the sextant stand, should be made, and it will be found 

 convenient to sink hollows in the top to correspond with the 

 three foot screws to prevent shpping. 



Other little hollows sunk in the top for the spare dark eye- 

 pieces to lie in, will also prevent these falling off, and by placing 

 them in regular order, any one can be at once picked up with- 

 out delay, when it is requisite to change them. 



Another similar stool for the observer will make him com- 

 fortable, a great point for good observing. 

 Observing A special fitting forming the top of the observing stool 

 deserves mention. It consists of two wooden discs in contact. 

 The upper disc is secured to a metal block, running on a screwed 

 bolt of very coarse pitch, fixed in a deep groove running across 

 the lower disc, working on bearings at each end and actuated by 

 a small handle. The upper disc carries the sextant stand ; the 

 lower disc revolves on a pivot passing through the centre of the 

 stool, and secured by a butterfly nut underneath. By turning 

 the handle, the upper disc is moved as required backwards or 

 forwards in the direction of the artificial horizon as the sun or 

 star moves in altitude. 



ARTIFICIAL HORIZON. 



The glass in the roof should be of the best quality, and the 

 faces of each plane accurately parallel. 



A wooden trough to place inside the iron one is a convenience, 

 as it raises the level of the mercury up to the height of the 

 lower edge of the glasses in the horizon roof, a consideration 

 where low altitudes have to be observed. The reduced area of 

 mercury will not matter when observing the sun. When taking 

 stars, the iron trough only should be used, as stars are more 

 difficult to pick up, and its larger area will facilitate operations. 



Three short wooden legs or buttons, fitted to the iron trough, 



will enable it to stand steadier on uneven ground than the four 



projections usually cast on the under side. 



Horizon In connection with this, an artificial horizon stand is very 



^*^ ■ useful. This consists of two iron plates ; the lower one has 



