992 ME. F. JENKIN’S EXPEEIMENTAL EESEAECHES ON THE TEANSMISSION 
It is certain that in the two cases named the received current did not vary 1 per 
cent. ; here therefore we find one positive limit to the rate of transmission. No doubt 
oscillations of considerably less than 1 per cent, might be observed and recorded by 
employing a more sensitive instrument and stronger batteries ; but we clearly see that 
the useful effect produced rapidly decreases as the number of signals per minute increases, 
and therefore that, however sensitive the instrument and powerful the battery, signals 
sent at more than a certain rate will fail to cause any appreciable useful effect at the 
receiving end ; and we may safely conclude that in all submarine cables there is a limit 
to the number of signals which can be sent per minute, a limit which cannot be exceeded 
by any ingenious contrivance. 
Fifty-six dots per minute were distinctly visible when sent through 1802 knots, and 40 
dots when sent through 2192 knots. The mean strength of the currents showing these 
dots is less than half the maximum permanent current, because of the time lost in 
moving the Morse key, as already explained. The curves showing these dots appear 
therefore below the dotted line representing the middle of the scale. 
As the speed of sending decreased, the amplitude of oscillation of the spot or varia- 
tion of the current increased, but the mean strength of the current remained nearly 
constant. 
When dashes were sent (^. e. when the length of the battery-contact was made twice 
as long as the earth-contact), the mean strength of the received current was much higher, 
being above the middle of the scale. 
The effect of dashes could thus, and thus only, be distinguished from that of dots sent 
at a lower speed. For instance, dashes sent through 1802 knots (Table VI.) when the 
metronome beat 84 caused a 6 per cent, oscillation, and dots sent when the metronome 
beat 60 caused very nearly the same oscillation; but the strength of the current due to 
the dashes varied from 53-|- to 59-|-, whereas that due to the dots varied from 43 to 49. 
In the corresponding curves the dashes appear above the dotted line and the dots below. 
This effect will be easily understood when it is remembered that while dots are sent, the 
end A of the cable is altogether in contact with the earth for nearly half of each minute, 
but when dashes are sent, it is in contact with the earth for only about one-third of each 
minute. 
WTien the dot and dash are sent alternately, the effects, differing considerably from 
those produced when each is sent continuously, can be best studied in the curves annexed 
to the Tables. 
The top of the dot curve is higher, and the bottom does not go so low as when dots 
only are sent, whereas the top and bottom of the dash curve are both lower than when 
dashes only are sent ; hit the bottom of the dash curve does not go so low as the bottom 
of the dot curve. This was seen in every instance, but was most apparent at the 
higher speeds. 
The strength of the received current due to the long battery-contact during the dash 
is naturally greater than that caused by the shorter contact during the dot ; and as the 
