468 



the quantity of inflammable gas amounts to one twelfth part of the 

 atmospheric air present, an explosion may take place. 



For the purpose of preventing such accidents, Dr. Clanny has con- 

 trived to insulate a candle, by water placed both above and below 

 the lantern in which it is contained. The air, which is intended to 

 support the flame, is supplied by means of a pair of common bellows, 

 by which it is forced through the water beneath the flame ; and it is 

 again emitted, after having supported the combustion, by a bent tube 

 that passes into water from the top of the lantern. 



In consequence of this arrangement, if the air of the mine becomes 

 liable to inflame, the explosion will be confined to the mere content 

 of the lantern, of which only a small part will be consumed, unless 

 the quantity of inflammable gas be very suddenly increased. 



This communication is accompanied by drawings of the lantern 

 and its parts in detail, whereby any workman may be enabled to 

 execute it according to the design of the author. 



On the Light of the Cassegrainian Telescope, compared with that of the 

 Gregorian. By Captain Henry Kater, Brigade-Major. Communicated 

 by the Right Hon. Sir Joseph Banks, Bart. K.B. P.R.S. Read 

 May 27, 1813. [Phil Trans. 1813,^. 206.] 



The author having remarked the performance of a Cassegrainian 

 telescope, made by a self-taught artist at Ipswich, to be superior to 

 what he believes is usually expected from telescopes of this construc- 

 tion, has been led to make a series of experiments on the compara- 

 tive illumination given by the Cassegrainian compared with that ob- 

 tained by the Gregorian construction. For though the Cassegrainian 

 form has been considered merely as the Gregorian disguised, and has 

 been rarely adopted, in consequence of its inverting objects, a supe- 

 rior power of illumination, if correctly ascertained to exist, may prove 

 a valuable property, in addition to its advantage of being considerably 

 shorter than the Gregorian. 



In the telescope first compared by Major Kater, the specula were 

 cast at the same time, in the same metal, and to the same pattern. 

 The magnifying powers of the two instruments were ascertained 

 by experiment to be very nearly equal, feat with a small excess on 

 the side of the Cassegrainian. The two telescopes were placed side 

 by side, and pointed to the same object, which was a printed card, 

 at the distance of fifty yards ; and as the brightness, as seen in the 

 Cassegrainian, was far superior, its aperture was first reduced by a 

 ring of pasteboard, and then gradually enlarged till the card appeared 

 equally bright through both telescopes. After the respective areas 

 of aperture in each telescope had been measured, with due allowance 

 for the light obstructed in each by the small mirror, that of the Cas- 

 segrainian was found to be to the Gregorian as 46 to 108, or 3 to 7 

 nearly. 



In the second comparison made by Mr. Kater, the Cassegrainian 



