516 Intelligence and Miscellaneous Articles. 
the image twice reflected from any given point will be displaced in 
a plane perpendicular to that of the circle, when B turns about its 
axis. If the circle is placed horizontally, all the objects on the 
same vertical plane will be seen to pass in succession. 
Now let us suppress the telescope, and replace it by a simple 
vertical slit placed between the eye and the axis of rotation of A; 
suppose, moreover, the mirror is silvered all over. By placing the 
eye at a convenient height in front of the slit, we can look directly 
at an object above or below the mirror (according to its height), 
and we can make it coincide with the image twice reflected of 
another object which may be at any height, since the movement of 
B may bring it vertically to the desired height. A reading made 
as with the sextant will give at once the angle of the two objects 
projected on the horizon, such as would be obtained with a grapho- 
meter. 
It follows from the preceding that the coincidence should be 
exact with the image of the axis of rotation or vertical edge of A, 
which would reduce to zero the field of the objects seen in B by 
double reflection ; but it is easy to see that by slightly inclining the 
visual ray so as to have a sufficient field, the error produced will 
be extremely small from the properties of minima, since we move 
but little from the position m which the error is completely nul= 
lified ; with a small instrument which is only 7 centimetres in the 
side, an operator who is but little practised may take, in a few 
seconds, angles with sufficient accuracy for most ordinary opera- 
tions; the approximation mainly depends on the horizontality 
of the instruments. Although the coincidence becomes more 
difficult and the exactitude less for very large angles, we may work 
directly up to 140°.—Comptes Rendus, April 27,1885. 
ON THE PRODUCTION OF ALTERNATING CURRENTS BY MEANS OF 
A DIRECT-CURRENT DYNAMO-ELECTRIC MACHINE. BY JOHN 
TROWBRIDGE AND HAMMOND VINTON HAYES. 
It is often desirable to transform a direct current into an 
alternating one for the purpose of obtaining electricity of high 
tension by means of a Ruhmkorff coil, for studying the effects 
of stratifications in vacuum tubes, or for employing alternating 
currents in the study of magnetism. The best way is undoubtedly 
to employ an alternating dynamo-electric machine, as has been 
done by Spottiswoode. When, however, only a direct-current 
machine is available, the following method can be employed :— 
The dynamo machine, if it is not a shunt-wound machine, is 
shunted by a suitable resistance. We have employed for this pur- 
pose thin ribbon-steel about 1:5 centim. broad and -01 millim. in 
thickness. The remaining portion of the current from the machine 
is conducted to two brass or copper segments a, a’, fig. 2. This 
current is led to the primary coil, for instance, of a Ruhmkorff coil 
from two other segments }, 6’. These segments are fixed upon a 
