MODERN SCIENCE. 225 



to science as regards atmospheric phenomena, such as 

 pressure, currents, temperature of the air at different levels, 

 and the characters of the different orders of clouds. 



1745 1807. Attwood invented (1783) ATTWOOD's 

 MACHINE for proving, by a falling weight, the laws of 

 accelerated motion. 



1745 1827. Volta invented the hydrogen lamp; he also 

 invented the ELECTROPHORUS (to excite electricity in small 

 quantities), a convenient substitute, on a small scale, for 

 the electric machine and its objects ; it offers a good instance 

 of the conversion of work into electro-potential energy. 

 Volta also invented the EUDIOMETER (to ascertain the purity 

 of the atmosphere, or the quantity of oxygen in any given 

 bulk of elastic fluid) ; he also invented the ELECTROMETER 

 (to indicate the presence, or measure the quantity and 

 intensity, of electricity) ; and finally (1800) he invented the 

 ELECTRIC BATTERY, partly with the object of disproving 

 Galvani's theory of animal electricity. For he argued that 

 the electricity detected by Galvani was produced by the 

 two dissimilar metals, and not by the frog's leg itself; 

 that it was therefore chemical, not animal, electricity. It 

 was in accordance with this reasoning that he made an 

 apparatus wherein two metals zinc and copper brought 

 into contact with moisture, and separated from other 

 substances, produced electricity. That apparatus was the 

 voltaic battery in its simplest form. The voltaic pile, 

 another form of the battery, is made up of alternate 

 plates of zinc and copper, with pieces of flannel between 

 them wetted with water and salt the top plate being 

 copper and the bottom plate zinc, both connected by 

 a wire, through which a current (or circuit) is produced; 

 or rather, we should say, two currents circulate, the positive 

 current passing from the copper to the zinc and the negative 

 current the opposite way, from the zinc to the copper. Now, 

 if the wire is cut in the middle and each point tipped with 

 a bad conductor such as charcoal, through which electricity 

 runs with difficulty, a spark will glow between the points, 

 and that spark will grow to a brilliant luminous arch of 

 flame (if the battery is a powerful one), and thus become 



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