Prof. Thomson on Rapid Signalling by the Electric Telegraph. 61 
which will give deflections, positive or negative, bringing one or other 
of these letters on the reflected scale into the centre of the field of 
view. But it would be bad economy to give the simple signals to 
rare letters, and to require double or triple signals for double and 
triple combinations of frequent occurrence. Besides, by the plans 
which I have formed, it will, I believe, be easy to make much more 
than thirteen different positive and thirteen different negative 
strengths of electric operation, giving unmistakeably different degrees 
of deflection; and if so, then many of the most frequent doubleand triple 
combinations, as well as all the twenty-six letters of the alphabet 
singly, might be made by simple signals. But it is also possible 
(although I believe highly improbable), that in practice only three 
or four, or some number less than thirteen, of unmistakeably different 
deflections could be produced in the galvanometer at one end by 
electric operations performed on the other extremity. If so, the 
whole twenty-six letters could not each have a simple signal, and 
double signals would have to be chosen for the less frequent letters. 
Experience must show what number of perfectly distinct simple 
signals can be made, and I have scarcely a doubt but that it will be 
much more than twenty-six. Then it will be easy to invent a letter 
code which will use these signals with the best economy for the 
language in which the message is to be delivered. Towards this 
object I have commenced collecting statistics showing the relative 
frequency with which the different simple letters, and various com- 
binations of simple letters, occur in the English language, and I must 
soon have information enough to guide in choosing the best code for 
a given number of simple signals. 
The investigation leading to a measurement of the electro-mag- 
netic unit of electricity in terms of the electro-static unit, published 
since the commencement of the present year by Kohlrausch and 
Weber, has given all that is required to deduce from Weber’s own 
previous experiments the measurement of the electric conductivity of 
copper wire in terms of the proper kind of unit for the telegraph 
problem. The data required for estimating the rapidity of action in 
a submarine wire of stated dimensions would be completed by a 
determination of the specific inductive capacity of gutta percha, or 
better still, a direct experiment on the electro-static capacity of a yard 
or two cut from the cable itself. I have estimated the retardations 
of various electric pulses, and the practicable rate of transmitting 
messages by cables 2400 miles long, and of certain ordinary lateral 
dimensions, on the assumption that the specific inductive capacity of 
gutta-percha, measured as Faraday did that of sulphur, shell-lac, &c., 
is 2, from which it probably does not differ much. These estimates 
have been published elsewhere (Athenzeum, Oct. 1856), and I shall 
not repeat them until I can along with them give a table of estimates 
for cables of various dimensions, with the uncertainty as to the 
physical property of gutta-percha either done away with by experi- 
ment, or taken strictly into account. 
