724 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



The anemometer proper is formed of a vane forgiving the direction 

 of the wind, and a Waltmann's windlass to indicate the velocity. 

 An azimuthal commutator, divided into eight sectors insulated from 

 each other, is in connection by eight wires ending in the eight sectors 

 on one side with one pole of the battery, on the other with the 

 receiving apparatus. Upon this commutator a piston rubber con- 

 stantly presses, which is directed along the axis of the vane, and 

 constantly establishes an intimate metallic contact between that axis 

 and the sectors. The axis being also in communication with the 

 other pole of the battery, it follows that the circuit is always closed 

 across the sector on which the rubber presses ; that is to say, precisely 

 in the direction of the wind. An electric communication of the same 

 kind is arranged between the windlass, the battery, and the indicating 

 apparatus. The latter is a cylinder moved uniformly by clockwork, 

 so as to turn once round in twelve hours and to advance alone its 



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axis by a definite quantity, say two millimetres at each revolution. 

 Eight electro-magnets, whose armatures are fitted with pencils, are 

 arranged facing the cylinder, and every time the circuit of any one 

 of them is closed, the corresponding pencil traces a mark on the 

 cylinder, against the surface of which the movement of the armature 

 presses it, the length of which mark indicates the duration of the 

 wind at the same time as its direction. 



The number of revolutions accomplished by the windlass is 

 indicated in a similar fashion, and hence the velocity of the wind is 

 regularly registered. 



Space fails us to describe with the necessary details, the baro- 

 metrographs, thermometrographs, and other registering meteorological 

 instruments, whose construction is based on the intervention of 

 electricity. It is sufficient here to have given a general idea of this 

 application, and we shall conclude by insisting on the importance 

 that this method of observation cannot fail to have for the progress 

 of the science. Various systems are now followed in the principal 

 meteorological observation at Kew, Greenwich, Brussels, Rome, Berne, 

 and Paris. When stations of this kind shall be distributed over all 

 the globe, on continents and islands, and a series of exact observations 

 can be made with the necessary care and continued through long- 

 years, we shall be able to establish formulae of greater and greater 

 rigour to represent the laws of the movements of the atmosphere 



