362 
POPULAR SCIENCE REVIEW. 
and round it, deflects the magnets and the mirror with them. 
A beam of light proceeding from a lamp is reflected off the 
mirror on to a screen, and moves When the mirror moves. The 
clerk at the receiving end patiently observes the motions of 
that spot of light upon the screen, and from them interprets 
the message. 
There have been recording instruments for land lines of 
great diversity and utility. There are instruments in use at 
present which record or write down the message by chemical 
action, which emboss it or daub it in ink, which print it in 
Eoman characters, and which can even reproduce a sketch of a 
criminal’s features. Of non-recording instruments, those gene- 
rally employed are the “ single-needle ” instrument, in which 
the right and left movements of an upright needle involve the 
message ; the 66 sounders,” in which two bells of different pitch 
are struck ; and a receiving machine, in which a moveable 
pointer indicates letters on a dial. But until lately the mirror 
galvanometer was the only instrument used upon long cables. 
The inventor of that invaluable current detector has, however, 
designed and patented a new self-recording instrument, destined 
to relieve the weary watches of the telegraph clerks, and to 
emancipate those 66 slaves of the lamp.” Sir William Thomson’s 
patent siphon recorder received its name from the pen which 
writes the message being a little siphon, which draws off ink 
from an ink bottle by one end, and squirts it upon a running 
strip of paper by the other ; but the essential principle of the 
instrument is that of the galvanometer, so to say, inverted. 
Whereas in the latter the coil of wire through which the 
current of electricity from the distant station travels is fixed, 
and the magnets moveable, in the recorder the magnets are 
fixed, and the coil moveable. Just a& Oersted found that 
every conductor conveying an electric current acts upon a 
magnet, so Faraday perceived that a magnet acts upon a con- 
ductor carrying a current. Since the earth behaves as a great 
magnet, and the animal body is a conductor, though — luckily 
for us in thunderstorms — a poor one, it is a fact that one 
cannot walk in an east-and-westerlv direction, so as to cut the 
lines of terrestrial magnetic force which run approximately 
north and south, without establishing a difference of u electric 
potential ” between his two extremities, which, if joined 
through a current indicator delicate enough, would give rise 
to an electric current from the one extremity to the other! 
On the other hand, if a current is sent through a person from 
his head to his feet or from his feet to his head, he will be 
subjected to a real mechanical impulse in a direction across 
the magnetic meridian, but all too feeble to be sensible. In 
the siphon recorder a coil of wire is suspended between the 
