



CHAP, x.] VARIOUS APPLICATIONS OF ELECTRICITY. 723 



they can only give single and isolated indications of elements ; they 

 in no sense solve the important problem of a continuous registration, 

 or one of a very short period, which should give, for example, the 

 curve of the variations of the temperature. 



The idea of substituting automatic registering instruments for 

 ordinary ones is not new. In 1782, Magellan invented a perpetual 

 meteorograph ; but it does not appear to have been used in practice. 

 The principle of that apparatus was purely mechanical ; that is to 

 say, it derived from the movement itself that was caused by the 

 variations of the elements, the force required for registering the 

 indications. Many registering apparatus have been and still are 

 founded on this principle, which have the merit of simplicity and 

 economy, but which unhappily fail through insufficiency on account 

 of the smallness of the force thus relied upon. 



Another system consists in employing photography ; that is to say, 

 in producing on sensitive paper the image of the level of the mercurial 

 columns of the barometer, thermometer, &c., enlarged by suitable 

 optical apparatus. This system is naturally more costly than the 

 mechanical one, especially as it is necessary to use clockwork to 

 give a continuous motion to the band of paper on which the photo- 

 graphic records are made. 



Lastly, there is a third system which employs electricity as the- 

 registering agent/ Telegraphic apparatus, especially the writing and 

 printing systems, enable us to conceive in what way the electro- 

 magnetic currents are employed for registering meteorological indi- 

 cations. For example, the index of the instruments is provided with 

 needles which pierce an endless band of paper whenever they are 

 set in motion by the armatures of the electro-magnets ; this happens 

 whenever there is a closing or interruption of the electric circuit. 

 A clock regulates the periodicity of these makings and breakings 

 of contact, at the same time that its wheel\vork causes the paper 

 on which the registry is made to move forward. 



We may cite some of the electro- magnetic registering apparatus 

 employed in meteorological observations. 



' The first anemograph constructed in France was invented by 

 M. du Montcel, afterwards modified by M. Salleron, and finally 

 introduced by P. Secchi in the great nieteorographic machine which 

 he exhibited in the Champ de Mars in 18G7. 



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