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The deflections thus produced constitute the signals. In this 
way from six to seven words are easily transmitted through 
each of the Atlantic cables in one minute. 
In order to understand how this method of signalling should 
admit of signals being sent in more rapid succession than is 
possible when they are produced in the usual way, by inter- 
mittent currents, we may avail ourselves of an illustration 
employed twelve years ago by Faraday, who, in describing 
some of the earliest observations on the effects of induction 
on subaqueous telegraph wires, compared its action to that 
of the air- spring of a force-pump, whereby the intermittent 
action of the piston is caused to produce a nearly uniform 
flow of water. Just as the air-chamber of the pump effaces 
the effects of the separate strokes of the piston more com- 
pletely the greater the volume of air which it contains : so, in 
a submarine cable, the effects of intermittent currents are more 
completely obliterated in proportion as the electrical capacity 
of the cable is greater. The condition of the cable when kept 
constantly charged, as in the method of signalling which has 
been last described, corresponds however to that of a force- 
pump into whose air-chamber so much water has been driven 
that the air in it is reduced to a small bulk, in which case it is 
evident that each stroke of the piston must produce nearly its 
full effect on the issuing jet of water. 
Although this article has already greatly exceeded its in- 
tended limits, we are unwilling to close without adding one or 
two remarks of a more general character. That the political, 
social, and commercial consequences of the completion of the 
Atlantic cable will probably be very great, is obvious to all ; 
how great they are likely to be is best known to those most 
-conversant with political, social, and commercial matters. 
But apart altogether from results which may be looked for in 
the future, the past history of this great undertaking may 
teach us lessons which are of themselves of no small value. 
On the one hand, it proves to men of science, in perhaps 
a more striking way than ifc was ever proved before, how 
fruitful even of purely scientific results may be the practical 
application of scientific principles, and how much Science her- 
self may often gain by listening to the claims of Practice for 
assistance. On the other hand, it ought to convince self- 
styled practical men, who regard improvements in the arts of 
life as the highest end of science, and who seldom look with 
much favour on scientific pursuits which have no obvious rela- 
tion to practical applications, that investigations, in which 
they see nothing more than the mere indulgence of scientific 
curiosity, are often those whose results are eventually found 
to be of the greatest practical importance. 
