17 Drummond Place, <1>
My Dear Sir,
I am sorry to say that your proposed solution of the problem of “ Mahomet‘s Coffin <2>” will not do. There must be no strings nor supports of any kind. As I told you some time ago it was proved (in 1839 or so) by Earnshaw <3> that under the action of forces varying inversely as the square of the distance, such as those between your magnet and each element of your electromagnets, there is no position of stable equilibrium: – the potential alwaysincreases in some directions and dimishes in others from every point not forming part of the attracting mass.
Thomson <4> has extended this proof to soft iron & all magnetic bodies: but he has shown that it does not hold for diamagnetic bodies, and consequently, were <illegible deletion> bismuth as powerfully diamagnetic as iron is magnetic, the ball of bismuth woudl be suspended, in stable equilibrium for all displacements, in the vertical axis of a hollow cylindrical coil of wire conducting a sufficiently powerful current. I have not opportunity just now to seek the reference for you, but it is in the Phil. Mag., <5> and Thomson has only three or four papers there whose titles involve the word “magnetism”, so that you will not have much difficulty in finding it.
I am looking forward to the receipt of your materials for my historical sketch of Spectrum Analysis <6> – My lecture is to be on May 15th. For the rest of this month, my address will beUnion Club,
next month Edinburgh as usual.
Would you send a short note to Prof. Balfour, <7> Inverleith Row, giving the titles of your mathematical papers, so that they may be laid before the council on Friday so as to be put in the billet for May 1st, when I shall be happy to take charge of them.
P. G. Tait.H. F. Talbot Esqre
2. It is said that Mahomet’s coffin, in the Hadgira of Medina, is suspended in mid-air without any support. Many explanations have been given of this phenomenon, the one most generally received being that the coffin is of iron, placed midway between two magnets. Burckhardt visited the sacred enclosure, and found the ingenuity of science useless in this case, as the coffin is not suspended at all.
3. Samuel Earnshaw, ‘On the nature of the molecular forces which regulate the constitution of the luminiferous ether’ reprinted in Cambridge Philosophical Society Transactions v. 7, 1842.
4. Sir William Thomson, Professor of Natural Philosophy at the university of Glasgow, later known as Lord Kelvin. Professor Thomson and Professor Tait wrote together Elements of Natural Philosophy (second edition 1879, Cambridge University Press).
5. Philosophical Magazine.
6. Tait could be referring here to his work Light (Edinburgh: A & C Black, 1884) or perhaps a project which he intended but never completed.
7. WHFT had already written to Dr John Hutton Balfour (1808–1884), Scottish botanist at his home in Edinburgh, on April 9, with these two titles ‘On Fagnani’s theorem’ and ‘On the Theory of Parallel Lines’.