204 INTERESTING SCIENTIFIC WORK IN THE ARCTIC 



treme accuracy in timekeeping, every known device being em- 

 ployed to insure absolute uniformity of running. They are set 

 to the exact time of a standard meridian (such as that of Green- 

 wich, England), several being carried, of which the exact rate of 

 gain or loss in each is known. By comparison of these with each 

 other the observer is able to tell the precise moment of taking 

 his observation. 



Particular care must be taken in the selection of the chronom- 

 eters. These instruments are subject to errors in timekeeping 

 under the most favorable conditions of handling and tempera- 

 ture. It is inevitable, during the hard knocks of a sledge jour- 

 ney in a temperature low enough to congeal the oil with which 

 they are lubricated, that they cannot be considered the instru- 

 ments of precision they are under better conditions. 



As their use is principally in determining the longitude, errors 

 in this direction are of little moment near the Pole. At the Pole, 

 where all meridians converge, one longitude is the same as an- 

 other, and every altitude is a "meridian altitude," which does not 

 require a chronometer for its solution. 



An artificial horizon completes the list of indispensable instru- 

 ments. The greatest difficulty in astronomical work in the Arc- 

 tic lies in getting a suitable horizon. The mariner in the open 

 sea in clear weather has little difficulty in measuring the angle 

 between any heavenly body and the actual visible sea horizon. 

 On shore or surrounded by a sea of ice in various forms there is 

 no such thing as a level, unbroken horizon, so that it becomes 

 necessary to resort to an artificial horizon of some sort. 



The usual method is to use a basin of mercury, which, when 

 sheltered from the wind by a glass cover, forms a perfectly 

 smooth horizon surface in which the heavenly bodies are bril- 

 liantly reflected. It is then necessary only to measure with a 

 sextant the angle between the body in the heavens and its re- 

 flection in the artificial horizon to determine its actual elevation 



