Pièce 20 - Letter from George Airy

4 pp.

Royal Observatory Greenwich - Gives an account of the difficulties involved in constructing a self-registering machine to measure terrestrial magnetism: 'There is only one way in which I can conceive the possibility of such a machine, namely by making the magnet carry a point, and constructing [a] mechanism which should lift the paper up to the point to receive a dot and then withdraw the paper that the magnet might quietly make up its mind as to the position that it would take for the next dot: this to be repeated as often as necessary, say every minute. Taking into account the horizontal oscillation, the up-and-down-bobbing &c of the magnet, which may be checked, I think this may be mechanically possible: but what shall the dot be?' - there are problems with the different possible inks. Is puzzled by David Brewster's latest work - 'it amounts to this, that light from different parts of the spectra can interfere. This is quite opposed to all analogy'.