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particular part, or to concentrate its force upon thesubject. Some have thought to effect this, by makingthe sides and dome of an elliptical or parabolic figure ;expecting, from the mathematical properties of thosefigures, that the rays of fire, striking all over the inter-nal surface, would be reflected back into a small focalspace, in the same manner as the solar rays are concen-trated by burning concaves. On the authority of those,who had recommended these kinds of furnaces, I pre-pared, many years ago, an elliptical one; which wasfigured with great care, not only in a moist state, but afterit had been dried and burnt, by the revolution of afemi-elliptical plane about its axis. In the effects of thisfurnace, I was greatly disappointed : for it could not beobserved, that any focal reflexion obtained in it; or thatany advantage resulted from the exactness .of its figuration,or from the precise species of its curvature. And indeedmany causes, which it is foreign to the present purposeto enter into a discussion of, concur in preventing both theregular reflection of the rays of heat in furnaces, and thecollection and union of those which have been so reflected.

Others have attempted this concentration of fire upona different principle. By blowing a stream of air throughthe flame of a lamp or candle, the flame, as already ob-served, is made to converge into a kind of focal point,and acts there with a great increase of its force : by mul-tiplying these streams of flame, or impelling a number offlames into one point, the heat may be augmented to avery high degree. In furnaces also, the flame, and themost considerable part of the heat, follow the directionof the current of air : and hence it has been proposedto impel streams of air, from different parts of the cir-cumference of the fuel, to the middle, by means of severalbellowfes placed round the furnace. Though the princi-ple,