Page:Proceedings of the Royal Society of London Vol 1.djvu/488

Rh 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 he 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.

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 Cassegraiuian 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, but 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 Cassegrainian 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 Casscgrainian