374 EXPEDITION TOWAED THE NORTH POLE. 



The variation of moisture in tlie atmosphere performs a very im- 

 portant part in all the meteorological changes. Its effects, however, 

 are probably less marked in the Arctic regions than in more southern 

 latitudes. The first effect of the introduction into the atmosphere of 

 moisture is to expand the air and to diminish its weighty but after an 

 equilibrium has taken place, it exists, as it were, as an independent 

 atmosphere, and thus increases the pressure. These opposite effects 

 render the phenomena exceedingly complex. 



Winds. — As to these the following observations are to be regularly 

 and carefully registered, namely: The average velocity as indicated by 

 Eobinson's anemometer; the hour at which any remarkable change 

 takes place in their direction; the course of their veering; the exist- 

 ence at the same time of currents in different directions as indicated 

 by the clouds ; the time of beginning and ending of hot or cold winds, 

 and the direction from which they come. Observations on the force 

 and direction of the wind are very important. The form of the wind- 

 vane should be that of which the feather part consists of two planes, 

 forming between them an angle of about 10°. The sensibility of this 

 instrument, provided its weight be not too much increased, is in pro- 

 portion to the surface of the feather planes. Great care must be taken 

 to enter the direction of the wind from the true meridian, whenever 

 this can be obtained, and in all cases to indicate whether the entries 

 refer to the true or magnetic north. Much uncertainty has arisen on 

 account of the neglect of this precaution. 



In accordance with the results obtained by Professor Coffin, in his 

 work on the resultant direction of the wind, there are in the northern 

 hemisphere three systems roughly corresponding with the different 

 zones, viz, the tropical, in which the resultant motion is toward the 

 west, the temperate, toward the east, and the Arctic, in which it is 

 again toward the west. 



In the discussion of all the observations the variation of the tempera- 

 ture and tlie moisture will appear in their connection with the direction 

 of the wind. Hence the importance of simultaneous observations on 

 these elements, and also on the atmospheric pressure. 



Precipitation. — The expedition will be furnished with a number of 

 rain-gauges, the contents of which should be measured after each shower. 

 By inverting and pressing them downward into the siiow, and subse- 

 quently ascertaining, by melting in the same vessel the amount of water 

 produced, they will serve to give the precipitation of water in the form 

 of snow. The depth of snow can be measured by an ordinary measuring- 

 rod. ]Much difficulty, however, is sometimes experienced in obtaining 

 the depth of snow on account of its drifting, and it is sometimes not 

 easy to distinguish whether snow is actually falling or merel}' being 

 driven by the wind. 



The character of the snow should be noted, whether it is in small 



