96 THE POPULAR SCIENCE MONTHLY 



horizon consists of a flat dish about three inches wide and five or six 

 long, filled to a small depth with mercury, the surface of which be- 

 comes perfectly horizontal. The image of the sun seen in the mer- 

 cury will be as much below the horizontal plane as the actual sun is 

 above it; and the angle between the sun and its image is twice the 

 altitude of the sun. Except in very quiet air, the surface of the mer- 

 cury must be protected from the wind by an accurately made glass 

 cover. 



The glass artificial horizon is a piece of perfectly flat dark glass, 

 which will absorb the light which enters it and only reflect from its 

 upper surface. It is provided with leveling screws and spirit levels so 

 that it can be made perfectly horizontal. It is used in exactly the same 

 way as a mercurial horizon. 



Each form has its advantages; the glass horizon is easily trans- 

 ported, and can be used at temperatures below the freezing point of 

 mercury (about 39° below zero Fahrenheit). On the other hand, it 

 requires very careful leveling, and is liable to be broken. The mer- 

 cury of a mercurial horizon is usually carried in an iron bottle; in 

 pouring it back and forth it might be spilled and lost ; and at very low 

 temperatures it would be necessary to heat it to keep it liquid ; but then 

 it immediately takes a level surface and requires no leveling. 



Lieutenant Shackleton, traveling over the Antarctic continent, de- 

 termined his position by means of a small transit. Commander Peary 

 and Dr. Cook, traveling over the floating ice of the Arctics, used sex- 

 tants. The former used a mercurial and the latter a glass horizon. 



It is interesting to note that if a man were taking an observation 

 standing, with the sun about 6 degrees above the horizon and the 

 artificial horizon on the level of his feet, it would have to be about 45 

 feet from him, and as he would look at it from an angle of about 6 

 degrees, it would only appear about half an inch long. If the altitude 

 of the sun were 12 degrees, the artificial horizon would be 25 feet away 

 and appear about an inch long. This can be easily imitated by putting 

 a sheet of paper on the ground and looking at it from distances of 25 

 and 45 feet. Under such conditions the difficulties of making a good 

 observation would be much increased. If, however, the artificial hori- 

 zon were raised on a support, the observer would stand much closer to 

 it, and the observation could be more easily made. 



Another important instrument is the chronometer keeping mean 

 Greenwich time; for, as has already been shown, the determination of 

 position in general requires a knowledge of Greenwich time, though 

 at the pole itself this is not necessary. Whenever an explorer remained 

 as long as a week in one place he should determine, as well as he could, 

 how much his chronometers were gaining or losing per day; and he 

 should be most particular to determine the changes in their errors, be- 

 tween the times of leaving and returning to his base station. 



