1 879.] Proceedings of Societies . 509 
endless variety of figures can be obtained. As in other harmo- 
nographs, a writing table, on which the paper is placed, and an 
aniline glass pen, are used. Several of the figures done also on 
glass were thrown on the screen, the stereoscopic effedt being 
very apparent. In reply to a query, Mr. Wilson said that he had 
adapted some of the figures to the stereoscope. 
Prof. Hughes explained his new Induction Balance, and 
showed some of its principal effedls. It is well known that on an 
eledlric current passing along a wire adjacent to another wire, an 
induced current is set up in the second wire in an opposite direc- 
tion to the first or primary current. In the induction balance 
two primary circuits or coils are taken, with the same current 
(interrupted by a microphone adted upon by the ticking of a clock) 
running through both ; and between these is placed a secondary 
circuit or coil in connedtion with a telephone. The primary 
coils are so wound as to oppose each other’s indudlion on the 
intermediate secondary. There is a point, moreover, between 
where these opposed indudtive influences exadtly neutralise each 
other. If the secondary coil be placed there no induced ticking 
of the clock can be heard ; but if the secondary be displaced to 
one or other side of this point the ticking can be heard in the 
telephone, increasing in loudness as the secondary approaches 
one or other of the primaries. If the distance between the pri- 
maries be graduated into a scale this contrivance becomes a 
sonometer, since it gives an absolute zero of sound and degrees 
of loudness. It is well adapted for measuring the hearing power 
of the ear, and when used for this purpose it is known as the 
audiometer. By splitting the secondary coil into two parts and 
placing each close to its proper primary, so that there are four 
coils in all arranged in two pairs, the extremes in one primary 
circuit and the means in one secondary, the two opposing parts 
of the balance can be separated from each other, so as not to 
disturb each other’s adlion, and the balance made very sensitive 
by the closeness of the primaries and secondaries. Prof. Hughes 
finds that there is a line or zone of maximum indudlion midway 
between each primary and its secondary. If a conductor, such as 
a piece of metal, be put in this position it has a maximum distri- 
buting effedl on the balance, due probably to the eledlric currents 
generated in it by the induction. The effedl is apparently propor- 
tional to the condudlivity of the metal. It requires an exadtly 
similar piece of metal put between the other pair of coils to 
restore the equillibrium of the balance. A very slight difference 
of alloy or of weight between two like coins is at once observed 
from the imperfedt restitution of the balance ; base coins are thus 
also at once detedled. Again, it is possible for a person to tell 
what particular coin or coins are in one part of the balance by 
trial of the same coins in his part. When plates of non-magnetic 
metals are held vertical in the balance their distributing effedt is 
nil ; iron, on the other hand, gives its maximum effedl at this 
