CHAP. I.] ARTIFICIAL HORIZON 15 



three short legs on which it stands firmly ; the upper one is 

 pierced by three long large-headed screws, which serve as legs 

 and fit into slight hollows on the lower plate. By adjusting 

 these, the horizon laid on the upper plate can be levelled, when 

 we have uneven ground. Four iron battens, screwed on to the 

 upper plate so as just to permit the horizon roof to fit inside 

 them, will prevent any wind getting to the mercury. 



The horizon cover should be marked at one end, or side, and Mark on 

 this mark should in most cases be in the same position with '*^®^' 

 regard to the observer. Of this more is said under " Observa- 

 tions." 



A new form of horizon is now being introduced, with the Amaiga- 

 object of diminishing the waves set up in mercury by Trough, 

 vibrations. 



It consists of a circular shallow trough, of metal gilt. This 

 is amalgamated, after getting the surface absolutely clean and 

 free from grease, by wetting it with a few drops of dilute sul- 

 phuric acid, and then rubbing into it a drop of mercury until 

 the whole surface is bright, when a very small quantity of mer- 

 cury added will flow evenly and form a horizontal surface. 

 The dross is wiped off with a broad camel's-hair brush. 



In this shallow trough waves are killed almost instantane- 

 ously. The trough should be thoroughly washed on each 

 occasion before being used. 



This form of artificial horizon has proved itself to be invalu- 

 able on shaky ground, and is supplied to all surveying ships. 

 The circular form has recently been abandoned, and the latest 

 pattern is rectangular in shape. Care is necessary to see that 

 the thin film of amalgamated mercury flows evenly over the 

 surface of the trough, and completely covers it. The trough 

 should be levelled by means of a spirit level before pouring on 

 the mercury. 



THEODOLITE. 



The less a theodolite is tampered with by unpractised hands 

 the better, but they must be adjusted from time to time, and 

 little things are constantly wanting attention. 



The adjustments are well described by Heather, but as 

 it is very important to know them, they are here given, 



