12 Mr Harris on Magnetic Intensity by the 
glass vessel v, Fig.'7, open at both ends, and having its lower 
edge ground to the pump plate, is placed round the receiver 7, 
Fig. 2. Two brass-tubes wy, (Fig. 7,) communicate with the space 
between the outer and inner receivers, one of them passing out as 
a syphon, the other being connected with a funnel; and thus a 
circulation of the fluid, placed round the receiver, may be conti- 
nued to any degree of temperature required. 
22. It is important to remark, that neither of the above me- 
thods of research can be relied on, unless the magnetic vibrations 
be observed in a very rare medium ; for currents are produced 
by the heated glass, the effects of which on the oscillations of 
the bar, mixed up with the influence of heat on the needle itself, 
are extremely anomalous. My friend Mr Fox of Falmouth, who 
has made some interesting experiments on this subject,* informs 
me, that he made the existence of those currents evident, by 
means of a little smoke introduced into the receiver. He says, 
that they were sometimes sufficient to impart motion to light 
needles, and cause them to vibrate in arcs of from ten to twenty 
degrees. During the state of vibration of other bars, the ares 
became sometimes more rapidly reduced, and in some instances 
the vibrations were greatly increased in number. Light needles 
were more disturbed than heavy bars. Fine wires not magnetic, 
and needles composed of non-metallic bodies, were similarly af- 
fected. The effects continued until a perfect equilibrium of 
temperature took place in the receiver, and were not apparent 
when it was exhausted of the contained air. 
These results, taken in connexion with the experiments above 
adduced (a), seem to confirm the necessity of a very rigorous 
method of experiment in deducing the rate of vibration, so as 
to exclude the disturbing influence of the atmosphere. 
23. Of the changes which ordinarily occur in the conditions of 
* Lond. and Edin. Journal of Science for October 1832, 
