from the sun and poured down upon the earth in one year would suffice to melt a sheet of ice thirty metres thick, and enveloping the entire globe.
About the year 1860, M. Mouchot, then Professor of Mathematics in the Lycée of Alençon, being stimulated by the researches of Pouillet as well as by those of Melloni, the ablest of Italian physicists, who has made experiments of incomparable precision upon the transmission of heat, boldly attacked the question of the utilization of the sun's heat. The mechanical equivalent of heat had at length been determined. Thanks to Melloni, we already knew the quantity of caloric which different bodies, as glass, when reduced to thin laminæ, suffer to pass through, as also the difference in the reflecting power of polished metallic surfaces according to the nature of the metals, employed. But to measure the amount of vis viva transmitted daily from the sun to the earth, and, more Utopian still, to concentrate, at little cost, the sun's rays, so as to realize all the effects of which they are capable, were objects the attainment of which was henceforth insured, though Buffon and Saussure had failed, owing to the insufficiency of the data at their command. The question is now merely a matter of calculation, an application of well-known physical laws.
In order to concentrate to any useful purpose the sun's rays, there was need of a receiver which should be of moderate size and reasonable cost. After sundry attempts, one of which was with an apparatus resembling that of Saussure, Mouchot contrived a vertical boiler of copper, blackened on the outside, covered with three concentric bell-glasses, and resting on some bad conductor of heat, as sand, brick, or wood. Soon he increased the power of his apparatus by the addition of a metallic reflector, which enabled him to dispense with two of the three bell-glasses. With this apparatus he considerably raised the temperature of the water in the boiler, reduced it to vapor, melted sulphur, the liquefaction temperature of which is 116° C., and after twenty minutes of insolation brought the empty boiler up to the temperature of 200° C.
With this reflector a few seconds suffice to set on fire a heap of shavings or a piece of board. In a glass vessel placed at the focus of the reflector and inclosed in another vessel of glass, one kilogramme of tin has been melted in two minutes; the same quantity of lead took five minutes, and of zinc, six. The fusion-point of these three metals is 235°, 335°, and 475° C. respectively. With spherical or parabolic mirrors, whose focus is a point, and not a line, as in the conical or cylindrical mirrors employed in the foregoing experiments, the concentration of solar heat would have been still stronger.
While engaged in these investigations, the ingenious experimenter brought out his Marmite Solaire, a cylindrical glass vessel, in which is placed another cylinder of copper or of wrought-iron blackened on the outside, and resting on the bottom of the glass receiver. The
