It is now more than thirty years since Commander W. E. May, R.N. (formerly of the Admiralty Compass Observatory), drafted this hitherto unpublished paper, recording his opinions based on a study of documents made available to him in 1947 by Messrs Kelvin, Bottomley and Baird. The documents referred to in the paper are:

The Thomson v. Moore case as presented to the House of Lords in the Thomson v. Moore case (Patent Design and Trade Mark Cases, Vol. VII, No. 36.)

The case of Thomson v. Hughes (Patent Design and Trade Mark Cases, Vol. VII, Nos. 9 and 22.)

Report of Proof of case of Kelvin v. Whyte Thomson &. Co.

Bound volume of patent specifications referred to in the last named.

It is a curious habit of editors and publishers to invite well-known persons to write articles on subjects outside their normal orbit. Thus in 1874 Sir William Thomson was invited to write for Good Words an article on the mariner's compass. He took up the task and soon realized that he did not know enough of the subject to complete the article. He then began to study the compass and the final part of the article was published in 1879. Such is one of Lord Kelvin's explanations of how he came to interest himself. In 1885, in an affidavit for the Moore case, he said that he took up the study of the compass in 1871, whilst elsewhere he said that it was the necessity of writing for the Royal Society an obituary notice on Archibald Smith, who died in 1872, which first turned his attention to compasses.