OF
A CENTRIFUGAL MIRROR,
To collect Solar heat.
My fiftieth and last [Plate] contains this idea: It is not intended to vie with the usual mirror, in correctness of form, or intensity of local effect—but to offer, by the largeness of it’s dimensions, some properties which better mirrors cannot present. It is intended to pave the way for the use of the Sun’s rays in Engines of Power. For this purpose, however, it must probably be transported to some tropical climate, where “a cloudless sun” diffuses it’s rays more constantly, and less obliquely, than in our northern climes.
This is the more necessary here, because this Mirror can only be used in a horizontal position, and is in fact a fluid Mirror. [Fig. 1], shews it mounted on a steady frame A B, and having a strong axis on which it can be turned, faster or slower, according to it’s dimensions; and it may or may not be floated on water, to lessen the stress on the axis. The Mirror, properly speaking, is composed of mercury—contained in the revolving vessel C D, whose motion should be given by proper machinery in the most uniform manner possible. The mercury, thus turned, acquires a concave surface, a, b, c; and receiving the parallel rays d c, e b, and, f a, collects them into the focus F; in, or near which, is placed the vessel where the effect is to become useful, and which of course is moveable so as to follow the sun’s motion. Those of my readers who have seen the machines used for fixing the sun’s image in the solar microscope, will be at no loss to conceive how our present focal station must be moved to adapt it to a fixed mirror. I shall only add further, that it is not necessarily an exact movement that is here wanted; since the vessel to be heated would have dimensions somewhat large, and the focus itself be only brought to a moderate degree of precision. In a word, the utmost heat wanted would be, what could be usefully employed in heating water. It remains then to be observed, that the source of power, in this Machine, is magnitude of parts, more than precision of form: yet it may be mentioned, that the form we thus procure in the revolving mercury, is a solid of revolution, having the logarithmic curve (a, b c) for it’s section—a curve, which in fact, comes indefinitely near to the parabolic figure which would be required, if greater precision were attempted. We finish then, by observing, that the bottom itself of the revolving vessel might be made concave, (like the dotted line under that a b c) in order to avoid the necessity of using a large quantity of mercury, to form the reflecting surface.
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.