| Description | (Postmarked 19 Feb.) (Not printed by Muirhead; see Introduction above.)
Dear Sir,
It was Cobalt ore, not Cobalt, I ought and meant to have asked you about. You must know that I doubt whether Cobalt is a distinct semimetal. I have made many experts wet & dry and abundance of others upon cobalt revived from Zaffri. None of these are fit to be told in post letters. I have the same suspicions about Nickel. Don't you talk of such things to the acute chymists you converse wth.
Nothing in the wheel has puzzled me more than the exclusion of the fluid-piston from the steam pipes. And at last, the method I have been obliged to take up with is not sufficiently simple. The rest is simple, & easy to be executed, requiring no cocks, nor any apparatus whatsoever, excepting the three internal valves. [There follows a sketch of the valves.] I see the drawing wont do, so you must wait, unless you can understand what follows. A circular plate of bell metal is to be fixed on the Axis of the wheel at aa. In this three steam pipes are to terminate open, comme ça [There follows a thumbnail sketch.] so that an equilateral triangle might be formed by joining their centers by right lines. This plate must be ground true to another plate perforated in the middle for the axis of wheel to pass thro, & near the edges in this manner [There follows a thumbnail sketch]. The upper & longest perforation is to communicate with the condenser. The lower with the boiler. Nothing more need be said to You. Newton's metal & similar metals I have taken some pains about, & can prevent the calcination of, I believe, in this machine. But I also believe the wheel will never turn very fast. By the way little credit is to be given to what books affirm abut metals. Those which are said to have the greatest repugnance may be mixed easily &c &c &c &c.
You must often have observed the apparent dancing of distant visible objects in the Spring in windy weather especially. If the weather is calm what would happen. But I have not leisure to write more about refractions at present. Huygens's swil hung hung in water, is secure from all errors from expansion & its opposite. Copal is readily dissolved here in linseed oil, so as to make a most beautiful & almost perfectly transparent varnish, but I know not the process, & have never endeavored to investigate it.
It is now late & I am monstrously fatigued. Success & happiness & no Ennui to you. Respects to Dr Roebuck to whom I have lately written.
Farewell.
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