312 Intelligence and Miscellaneous Articles. 



If the polarization observed be due to the reflexion of the rays of 

 the sun by the comet or its envelopes, small stars will be seen more 

 distinctly through it when the polarized light is extinguished by the 

 application of a Nicol's prism. 



Whilst I was investigating the polarization of the atmosphere, I 

 observed the remarkable fact, that when objects situated far oiF in 

 the open country are rendered indistinct by the interposition of a 

 light mist, a part of their distinctness may be restored by viewing 

 them through a Nicol's prism, which extinguishes all the light po- 

 larized by the mist in a plane passing through the sun, the object, 

 and the eye of the observer. The objects thus rendered more distinct 

 and visible were seen through that portion of the mist in which the 

 polarization of the light reflected by them was at its maximum. 

 This method of rendering visible objects rendered indistinct by fogs 

 or mists may, it appears to me, receive important applications in 

 military and naval operations. — Comptes Rendus, February 21, 1859, 

 p. 384, 



NEW APPARATUS FOR OBSERVING ATMOSPHERIC ELECTRICITY. 

 BY PROF. W. THOMSON, 



I have had an apparatus for atmospheric electricity put up on the 

 roof of my lecture room, and got a good trial of it yesterday, which 

 proved most satisfactory. It consists of a hollow conductor sup- 

 ported by a glass rod attached to its own roof, with an internal 

 atmosphere kept dry by sulphuric acid : the lower end of the glass 

 rod is attached to the top of an iron bar, by which the hollow con- 

 ductor is held about two feet above the inclined roof of the building. 

 A can, open at the top, slides up and down on the iron bar which 

 passes through a hole in the centre of its bottom, and, being sup- 

 ported by a tube with pulleys, &c. below, can easily be raised or 

 lowered at pleasure. A wire attached to the insulated conductor 

 passes through a wide hole in the bottom of the can, and is held by 

 a suitable insulated support inside the building, so that it may be 

 led away to an electrometer below. To make an observation, the 

 wire is connected with the earth, while the can is up, and envelopes the 

 conductor — its position when the instrument is not in use. The earth 

 connexion is then broken, and the can is drawn down about eighteen 

 inches. Immediately, the electrometer shows a large effect (from 5 

 to 15 degrees on my divided ring electrometer, in the state it chanced 

 to be in, requiring more than 100 degrees of torsion to bring it 

 back to zero, in the few observations I made). When the surface 

 of the earth is, as usual when the sky is cloudless, negative, the 

 electrometer shows positive electricity. But when a negative cloud 

 (natural, or of smoke) passes over, the indication is negative. The 

 insulation is so good that the changes may be observed for a quarter 

 of an hour or more ; and when the can is put up, the electrometer 

 comes sensibly to zero again, showing scarcely any sensible change 

 when the earth connexion is made, before making a new start. — 

 Extract of a letter to Dr. Joule in the Proceedings of the Manchester 

 Philosophical Society. 



