722 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



or trembler. Lastly, the regulation of the energy of the current, 

 and thence of the shocks, is made in the following manner : 



Each bobbin is wrapped in a covering of copper, which may be 

 moved by a screw on the outside so as to increase or diminish at 

 pleasure the length of the parts of the coil that are covered. by this 

 kind of muff. Induced currents are developed in the copper outside 

 the bobbins, and since these currents are in an opposite direction to 

 those traversing the wires of the coil, they partly neutralize each 

 other. The experiments may then be commenced with very weak 

 currents at first, then stronger by degrees, up to the maximum of 

 energy, which is when the coils are entirely uncovered. 



Drs. Duchenne (of Boulogne), Tripier, and several manufacturers, 

 Messrs. Gaiffe, Trouve, Siemens and Halske, &c., have invented 

 electro-medical apparatus, into the description of which it would be 

 too long to enter, as our present object is simply to give an idea 

 of this special application of electricity. 



II. ELECTRICITY APPLIED TO METEOROLOGICAL OBSERVATIONS. 



Meteorology is a science which in many respects is still in its 

 infancy, a statement which will not appear astonishing to those who 

 take account of the infinite complexity of the phenomena it purposes 

 to study. The elements of these phenomena are manifold; the 

 atmospheric pressure, the temperature of layers of the air at different 

 heights, the temperatures of the soil and of the waters, hygrometry, 

 the force and direction of the winds, the amount of rainfall, are so 

 many facts which must be collected at as large a number of points of 

 the earth's surface as possible, and which require of observers, in order 

 to register all their variations, most laborious and painful assiduity. 

 Those too who devote themselves to this task are generally obliged 

 to confine their observations to fixed hours of the day and night, 

 whence result many inevitable but deplorable gaps. 



Attempts have been made for a long time to remedy this insuffi- 

 ciency of the means of observation, by inventing instruments to 

 leave automatic traces of their indications, and thus to dispense with 

 the immediate or direct intervention of the observer. Maximum and 

 minimum thermometers are examples of this sort of instrument, but 



