468 Miscellaneous Intelligence. 



2. On a New Optical Property ofDichroite. — This mineral, already 

 remarkable for the mimerous colours which it exhibits when exposed 

 to light, and more particularly to that which has been polarised, is 

 rendered still more ^o by a new property which M. Marx has 

 discovered in it, — namely, the power of polarising ordinary light. 

 M. Biot first observed this property in transparent tourmaline, and 

 afterwards used two plates of this substance, cut parallel to the axis 

 of the crystals, and placed one upon the other at right angles, in his 

 investigations of the optical properties of other minerals. Plates 

 of dichroite may be used for the same purpose : those specimens of 

 the mineral which come from Bodenmais are very proper for such 

 uses. According to M. Marx, the- curious -fact occurs, that the 

 plates may be cut from the crystals in a direction either parallel or 

 perpendicular to the axis. M. Marx thinks that the dichroite does 

 not belong to the rhomboidal system, and also that it possesses two 

 optical axes, the angle formed by them being divided by the axes of 

 crystallization. — Zeitzchriftfur Mineral, 1826. 



3. Velocity of Cannon Balls. — Lieutenant Helwig, of Prussia, 

 has invented a process for measuring the time occupied by a ball or 

 bullet in passing through a certain space. His process consists in 

 making the ball liberate the works of a time-keeper at the moment 

 when it quits the mouth of the piece, and in making it also stop the 

 time-keeper at the moment when it strikes an obstacle. The 

 numerous experiments which he has made already, offer interesting 

 results. He finds, for instance, that a light body, of the same 

 calibre with the bullet, moves, at the commencement, with much 

 greater velocity than the latter, equal charges being used: he finds 

 also, that small bodies move more promptly, a circumstance which 

 causes a considerable deviation of the ball, when there is sand or 

 any light body within the piece used. — Bnl. Uriiv. Hi iv. p. 119. 



4. Security from Fire in Theatres. — At the time of re-constructing 

 the theatre, I'Odeon, after its destruction by fire, it became a question 

 whether a metallic screen should not be interposed at the proscenium 

 between the public part and the stage, for the purpose ot preventing 

 the rapid communication of the combustion of the parts behind the 

 scene, or upon the stage, to the body of the theatre. M. D'Arcet 

 strongly opposed the use of an impermeable screen, but recom- 

 mended one of wire, and founded his opinion on the following fact, 

 which, if generally known, might tend to give a degree of self- 

 possession to the public, in a theatre, should they ever be called 

 upon to act in such a critical situation. " I had occasion, accu- 

 rately, to observe the first destruction of I'Odeon, by fire, in the 

 year 1799. I then remarked that the fire spread rapidly behind the 

 scenes, but without any smoke appearing in the body of the theatre; 

 and I was able to remain, for a long time, in one of the second tier 

 of boxes on the left side, without being inconvenienced by the he^it. 



