422 
POPULAR SCIENCE REVIEW. 
current is indicated by a movement of the spot of light, and 
these movements constitute the signals in which the messages 
transmitted through the cable are spelt out. 
From all that has been said hitherto in this article, it might 
be supposed that, if the near end *of the cable were put into 
connection with the battery, and the connection maintained 
unbroken for a definite time, as, for instance, one minute, a 
similar current (only slightly weaker in consequence of the 
never quite perfect insulation of the conducting wire) would 
appear at the far end, either simultaneously or after a certain 
interval, and after continuing at a constant strength for one 
minute, would suddenly cease. If this were the case, the spot 
of light of the reflecting galvanometer would move right or 
left along the scale through a certain number of divisions, would 
remain stationary for one minute, and then come back to the 
middle of the scale. And this is exactly what would occur, but 
for the effects of induction that have been already alluded to. 
The most familiar example of electrical induction is presented 
by the common Leyden jar. The charge of electricity, which 
the inner coating of this apparatus can take up from a given 
source of electricity, is, as is well known, many times greater 
than the charge which it would be capable of receiving from the 
same source if the outer coating were removed. Thus, in order 
to charge a Leyden jar to the highest point that is possible by 
means of a given electrical machine, four or five complete revolu- 
tions of the machine may be needed, whereas, if the outer 
coating of the jar were away, half a turn of the same machine 
might suffice to charge the jar to the same extent. That is to 
say, the quantity of electricity which the inner coating of a 
Leyden jar can take up, before the “ tension,” or tendency of 
the electricity to escape, rises to any given amount, is much 
greater than the quantity which would charge it up to the 
same tension if the outer coating were removed : in other 
words, the presence of the outer coating greatly increases the 
capacity” for electricity of the inner coating. 
Precisely the same kind of action takes place in the case of 
a telegraph cable immersed in the sea. Such a cable is exactly 
comparable with a Leyden jar, the surface of the conducting 
wire corresponding to the inner coating of the jar, and the 
water in contact with the outside of the insulator corresponding 
to the outer coating. Each of the cables which now lie 
between Yalentia and Newfoundland accordingly represents 
a Leyden jar whose inner coated surface measures about 
425,000 square feet. The way in which these facts affect the 
transmission of signals through the cables it is not easy to 
explain with perfect clearness, but it is hoped that the following 
considerations will make it at least partially intelligible. 
