Oscillations of the Horizontal Needle. 23 
brations were produced in the way already described (29), by 
means of two parallel threads of suspension. In this instance 
the same results ensued precisely. In regard to the differences in 
the are of vibration, these nearly vanished in a void ; the subse- 
quent retardation in the sunshine, however, was not so apparent 
as with the magnetic bar, although it was still very evident. I 
do not mean to adduce this experiment as being strictly compa- 
rative in regard to the magnetic bar, the rate of vibration having 
been found different, but merely in evidence of the effects pro- 
duced by the presence of a dense medium, when acted on by the 
sun’s rays. 
(f) The times, arcs, and vibrations of a magnetic bar having 
been noted im vacuo, as in the previous experiments, it was com- 
pletely exposed for twenty-five minutes to the intense light 
evolved by lime, under the flame of the hydro-oxygen blowpipe, 
and the oscillations taken again, but no differences whatever 
were found to ensue in the state of vibration. 
The first experiment (e) would seem to show, that the phe- 
nomena of oscillation observed in the sun’s rays, Table I, are not 
altogether dependent on magnetism ; the second (f), that mere 
light is, to a certain extent, incapable of producing them. 
87. The electricity pervading a vacuum, seems to have no 
appreciable effect on a magnet, oscillating under an exhausted 
receiver. With a view of examining this, I resorted to the fol- 
lowing experiment :— 
(g) A stream of common electricity, generated by a power- 
ful plate-machine of 3 feet in diameter, was caused to pass 
through a long receiver s, Fig. 2, placed within 18 inches of the 
needle. This receiver was about 6 feet in length, and 4 inches 
in diameter. The electrical light was continuous for twenty mi- 
nutes ; it seemed to spread over the whole interior surface of the 
receiver, presenting the appearance of a solid column of white 
light. No appreciable difference, however, could be discovered, 
