EXPEDITION TOWARD THE NORTH POLE. 373 



mometer with a guard to obviate the pressure of the water. As the 

 tendency, on account of the revohition of the earth, is constantly to 

 deflect all currents to the riglit hand of the observer looking down 

 stream, the variations in temperature in connection with this fact may 

 serve to assist in indicating the existence, source, and direction of cur- 

 rents. 



The depth of frost should be ascertained, and also, if possible, the 

 point of invariable temperature. For this purj)ose augers and drills 

 with long stems for boring deeply shouhl be provided. 



Fressure of air. — A series of comparative observations should be 

 made of the indications of the mercurial and aneroid barometers. The 

 latter will be affected hj the variation, of gravity as well as of temper- 

 ature, while the former will require a correction due only to heat "and 

 cai)illarity. 



As it is known that the normal height of the barometer varies in dif- 

 ferent latitudes, accurate observations in the Arctic regions with this 

 instrument are very desirable, especially in connection with observa- 

 tions on the moisture of the atmosphere, since, to the small quantity of 

 this in northern latitudes the low barometer which is observed there 

 has been attributed. 1 think, however, it will be found that the true 

 cause is in the rotation of the earth on its axis, which, if sufficieutly 

 ra})id, would project all the air from the pole. 



In the latitude of about GO there is a belt around the earth in which 

 the barqpieter stands unusually high, and in which violent fluctuations 

 occur. This will i)robably be exhibited in the projection of the curve 

 representing the normal height of the barometrical column in diiierent 

 latitudes. 



Moisture. — The two instruments for determining the moisture in the 

 air are the wet and dry-bulb thermometer, and the dew-point instru- 

 ment, as improved by Kegnault. But to determine the exact quantity 

 in the atmosphere in the Arctic regions will require the use of an aspi- 

 rator, l)y \vhi(;h a given quantity of air can be passed through an ab- 

 sorbing substance, such as chloride of calcium, and the increase of 

 weight accurately ascertained. It may, however, be readily shown that 

 the amount is very small in still air. 



A wind from a more southern latitude will increase the moisture, and 

 may give rise to fogs. Sometimes, from openings in the ice, vapor nniy 

 be exhaled from water of a higher temperature than the air, and be 

 immediately precipitated into fog. 



The inconvenience which is felt from the moisture which exhales with 

 the breath in the hold of the vessel may, perhaps, be obviated by adopt- 

 ing the ingenious expedient of one of the Arctic voyagers, viz, by 

 making a number of holes through the deck and inverting over them a 

 large metallic vessel like a pot. The exterior of this vessel being ex- 

 posed to the low temperature of the air without would condense the 

 moisture from within on its interior surface, and thus serve, on the 

 principle of the diffusion of vapor, to desiccate the air below. 



