FORECASTING THE PROGRESS OF INVENTION. 311 



mersed, a current will flow. If in a solution two metals that are 

 not acted upon are immersed, a current will not flow in a wire con- 

 necting them. If into this solution pieces of metal or other sub- 

 stances that will be acted upon are dropped, no current will be gen- 

 erated, because the chemical action takes place between substances 

 one of which is not in the electric chain. Coal is not a conductor 

 of .electricity, in the practical sense ; therefore it can not be used 

 directly in the electric circuit, even if we could find a way to oxi- 

 dize it satisfactorily; hence the only probable way of solving the 

 coal- battery problem is by some indirect process, and this may 

 introduce complications great enough to entirely offset all the 

 advantages. 



The belief that great development will be made along the lines 

 discussed in the foregoing is confined to those who possess some 

 familiarity with scientific matters, but the general run of intelli- 

 gent people only have a vague idea of what may be expected from 

 these sources, and the pictures drawn by their imagination in rela- 

 tion thereto are decidedly hazy; with them the greatest of all 

 future achievements is the solution of the problem of aerial navi- 

 gation. This belief is undoubtedly due to the fact that the theo- 

 retical limitations are not understood, or are not taken into consid- 

 eration, and as a consequence the average conception of a perfect 

 air ship, as well as its movements and velocity, is very different 

 from the actual possibilities. 



The most striking difference between imagination and possi- 

 bility, in this line, is perhaps in the relation between the size of 

 the ship and its carrying capacity, the latter being always greatly 

 magnified. An examination of any considerable number of the 

 illustrations of flying machines would show this point very forci- 

 bly. In many of these pictures the force of gravity is treated with 

 the utmost contempt, the ship being made apparently of sheet iron, 

 very similar in shape to a submarine torpedo boat, the sustaining 

 power being obtained by means of one or more moderate- sized pro- 

 pellers mounted upon vertical shafts, or else equally small aero- 

 planes. In those designs that display a greater regard for the 

 laws of Nature, the disparity in the proportions is not so great, but 

 in all of them it is very decided. 



It is evident that with our present knowledge of science there 

 are only two ways in which an air ship can be kept afloat, one 

 by the use of a balloon and the other by means of aeroplanes. 

 In the former the sustaining capacity is small relatively to the 

 volume, being about one pound for every fourteen cubic feet ; and 

 with the latter it is small relatively to the surface, being probably 

 not over one pound to the square foot. From this it can easily be 

 seen that the carrying capacity, even of a craft of large dimen- 

 sions, must be small, very much smaller than the popular notion 



