102 2Tecltanies, 2hysics, and Chemistry.

other. As the apparatus is very sensitive, these are rather tendencies 'co movement or change, than aetu'~l changes. I f an accident should happen, if one or both of the carbons should break, the current is sud- denly interrupted, the magnet ceases to act, the oscillator rises, and aUows the train to start and replace the carbons in their proper posi- tions. The pl'ty of this app:~ratus is so steady and easy that it will "~'ork with the alternating currents fi'om the electro-magnetic machine. ~he lamp is lighted or extinguished the moment the current is estab- llshed or interrupted. The lamp, therefore, becomes very suitable for telegraphic sign'ds or for llght-houses, in which, by these means, the ttashes may be obtained without any mechanism for rotatiom~Academy of Seiences of l'aris.~(/osmos.

Mode of Etddng on Glass. A painter at I~ourg-en-Bresse in France, reports to the Cosmos the

following singular and w~luable experiment: He traced upon a piece of glass a design in whitedead mixed with linseed-oil, and when it had dried, he laid it for a few minutes over a vessel in which hydro-fluoric was generating. When the glass was removed, it was found that the parts of the glass covered by the white paint were etched, while those uncovered had not been attacked. Should this observation be con- firmed, it would seem to open ~ new field to glass-etchers.

.Loss of Light 5y Glass Shades.* By WIL~A~ KI~o.

S~r~ :--Having recently tried some experiments, for the purpose of ascertaining the amount of light lost by the use of various deserlp- tions of glass shades, I thought that the results obtained might not wove uninteresting to some of your readers, more especially as it is ~. subject of practical import~nc% and does not seem to have,, attracted the notice which it deserves.

The following table exhibits the amount of light lost by the use of the various shades therein enumerated : -

Description of shade. Loss of light. C l e a r g l a s s , , , . 1 0 ' 5 7 p e r cent . G r o u m l g l a s s ( e n t i r e s u r f a c e g r o u n d ) , o . 2 9 . 4 8 " S m o o t h o p a l . . 5 2 " 8 3 " G r o u n d o p a l , . . . . 5 5 8 5 " G r o u n d o p a l , o r n a m e n t e d w i t h p a i n t e d f igures , t h e f i g u r e s i n t e r v e n i n g

b e t w e e n t h e b u r n e r a n d the p h o t o m e t e r sc reen 9 , 7 3 " 9 8 "

As the large amount of light lost by the use of a clear glass shade excited some sm'prise, a sheet of common window glass was place4 between the burner and the photometer screen, when it was found that 9.34 per cent. of the light was intercepted, thus confirming the result obtained by the employment of a shade of clear glass.

I may state that the shades were selected from a large number, and great pains were taken to obtain an average specimen of each kind.

Liverpool i fob. 2-tj lit60. *l~rom the Loud. ,)'our. of Gas I.Ighting~ 1:c., No. 192.