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19. Second Mirror



OF A SECOND MIRROR, For collecting the Sun's rays.

This Mirror seems superior to the former, as depending on fixed materials. It likewise, produces the desired effect, by offering a very large surface to the sun, and directing the rays to a focus, nearly enough to give the heat required for water, as before mentioned.



To do this, a frame A (Plate 50, fig. 2) holds the Mirror; and this frame has a horizontal motion round the post B, something like a common windmill. In this frame and on two horizontal trunnions, turns the Mirror C D; and one or both these trunnions are hollow, to admit of a process we shall shortly mention. This Mirror itself is composed of an air-tight ring C D, of a width proportionate to the diameter adopted; and on which are fixed two heads, much like those of a tambourine, (or the under head might be made of some metallic substance). The head a b c, is made of a fine texture, duly prepared and varnished till it becomes air tight, and then there are stuck to it, a number of small hexagonal looking-glasses or mirrors of any kind, (see fig. 7) which thus fill up the whole space, and prepare the Mirror for the intended change of form. The method of giving this form, consists in exhausting, more or less, this tambourine of air, when, by the pressure of the atmosphere, the heads will take the form a b c, that is a spherically concave form--fit to reflect the sun's rays as correctly as this our object requires; and thus may some thousand small images of the sun be brought to fall on the same spot, and an immense heat be occasioned. The accounts we have of the destruction of the Roman fleet by the united mirrors of Archimedes, make this process appear the more feasible--as whatever were the methods of uniting the foci of his mirrors, a similar effect may be expected from this simple process.

My readers will perceive that this Machine has the advantages of the universal joint, by which it can be directed to the sun in every position; and even made to fix his ardours on any immoveable spot for a good length of time. The persons to whom I particularly address these ideas, will require no further details to conceive the less obvious circumstances of this Invention. In general, we want no effect that requires optical precision; but if we did, it could be obtained to a good degree, by methods similar to these.

I shall only add here, that this fig. 2 is given as a section--because intended to represent a parallelogram, as well as a solid of revolution: and thus (with proper mirrors) to make what now appears a spherical focus, a linear one--fit to heat a cylindrical vessel with it's contents; and thereby draw power from the sun's heat, without running expense. I am serious when I say, that we can thus, practically, collect the solar rays which fall on many hundred square feet of surface; and produce by them, at any desired distance, effects to which those obtained from modern burning mirrors, are but as sparks to a blaze.