426 
POPULAR SCIENCE REVIEW. 
and the electrical properties of its materials are known, it is 
possible to calculate beforehand what dimensions must be given 
to the conductor and insulator in order to obtain any given 
rapidity of signalling. 
As to the absolute rate of signalling which is attainable with 
the Atlantic cable, the writer is informed, by Mr. Fleeming 
Jenkin, that, as the result of a calculation in accordance with 
the principles laid down by Professor Thomson, it appears that 
alternate battery- and earth-contacts at one end of the cable 
would produce a current at the other end, showing no percept- 
ible variations of strength, if they followed each other at a 
greater rate than about twenty-five double-contacts in a second; 
while battery-contacts maintained for about four-tenths of a 
second, and alternated with earth- contacts of the same length, 
would produce sensibly the greatest possible amount of varia- 
tion in the current at the receiving end. This shows, on the 
one hand, that legible signals could not be sent, in the way 
described, so fast as 25 per second ; and, on the other hand, that 
the distinctness of the signals would not be increased by send- 
ing them more slowly than at the rate of one in about eight 
tenths of a second. 
It is possible that to some readers these may appear to be 
matters of purely scientific interest, rather than of practical 
importance. Very little reflection, however, will show that, 
next to being able to transmit signals through a cable at all, 
the rate at which they can be sent is the point which most 
directly bears upon the commercial success of submarine tele- 
graphy. When it is remembered that each word of a message 
represents, on an average, sixteen signals sent and received, 
it will be understood at once that a very small diminution in 
the time occupied by a single signal is equivalent to a consider- 
able increase in the speaking power 33 of the cable; while, 
if once a telegraph cable is fully charged with messages, its 
speaking power is only another name for its money- earning- 
power. Hence the great practical importance of any instru- 
ment or contrivance by which the number of signals that can 
be sent in a given time is increased. 
The most obvious method of increasing the rate of signal- 
ling is by the employment of a receiving instrument capable 
of exhibiting the most minute variations in the strength of 
the received current ; for, as we have seen, small variations of 
the current can be made to succeed each other more rapidly 
than large ones. Herein is one among other important ad- 
vantages resulting from the use of an instrument of such 
extreme delicacy as Professor Thomson^ galvanometer. With 
this instrument nearly two words and a half per minute were 
obtained through the old Atlantic cable laid in 1858, whereas 
