8 REPORT 1851. 



secution of these experiments. I first of all ordered a series of thermometers from 

 Fastre of Paris. I had next to devise a suitable heating apparatus ; and finally I 

 had malleable iron bars made, my experiments having as yet been confined to that 

 metal. The mode of conducting the experiments, and of deducing results from them, 

 I shall not now enter upon ; because, although the progress of the experiments has 

 perhaps been as great as the difficulties of the inquiry and my very limited leisure 

 entitled me to expect, I think it very desirable to repeat and extend them further 

 before drawing any positive conclusions. I may state, however, that my method has 

 been communicated to Mr. Airy, the President elect of the Association, to Professor 

 Kelland, and to another scientific friend, and I have received every encouragement 

 from these gentlemen to proceed in the inquiry, which will I hope be considered by 

 the General Committee as a proof that these experiments have not been lightly un- 

 dertaken. 



On these grounds I request a renewal of the grant of 50Z. for the ensuing year, 

 although it is not at all likely that the whole, or even two-thirds of that sum, should 

 be required. Of the 50l. granted last year, I have spent only 201. Is. Id., of which 

 a detailed account has been handed to the General Treasurer. Some smaller expenses 

 are still unpaid. Messrs. Napier, the eminent machine-makers, have generously 

 supplied two admirably made bars, free of all cost. 



Letter addressed to Lieut.-Col. Sabine, R.A. (General Secretary/), 

 hy Captain Edw. J. Johnson, R.N.., F.R.S. 



This communication was made in a letter to Col. Sabine, of which the following 

 extract gives the substance : — 



You will perceive by the deviation tables of H.M. ships Ajax and Blenheim*, 

 that if no heed were taken of the deviation when regulating the ship's course, the 

 most serious consequences might be apprehended. 



Taking as an example the case of the Ajax, with the funnel J«p, running upon 

 an easterly course at the rate of 9 knots per hour, a simple diagram will show 

 that in 24 hours only, if no allowance were made for deviation, the ship would be 

 50 miles out of the reckoning ; and with the funnel doixn, the error would be increased 

 to 72 miles in the same space of time, while the case of the Blenheim would not be 

 very different. 



In the humid and misty atmosphere which so often prevails on the coasts of the 

 British Isles, the fact that a ship such as the Ajax, if steered a compass course (but 

 without allowing for deviation) for mid-channel between Ushant and the Lizard, 

 would instead thereof be running for the dangers about Ushant, with the funnel wp, 

 and with it down, be so far out of the proper course as to be advancing towards the 

 rocks south of Douarnenez Bay, is I conceive a proper example to show the import- 

 ance of attending to the effects produced upon the compass under the two conditions 

 of the funnels of steam ships. 



But besides the practical question, I wish you to bring under the notice of the 

 Section, the following results which I obtained with reference to the effect of hollow 

 iron cylinders upon the compass, when placed inside of each other; the object being 

 to ascertain whether the whole difference of deviation, under the two conditions of 

 the^e telescopic funnels, was due to the difference of their elevation and depression 

 only, or whether a portion of the said differences was attributable to the induced 

 magnetism of the separate parts of the funnel, when lowered, acting upon each 

 other. 



As it would have required more time than could be afforded to hoist the parts of 

 these huge funnels in and out of the ship while the requisite successions of observa- 

 tions were made, I procured three hollow iron cylinders, of smaller dimensions, their 

 several diameters being such as to admit of one cylinder being placed inside of 

 another, and leaving a space of about an eighth of an inch between their surfaces. 



Having placed a standard compass on one of the pedestals in the observatory, and 

 ascertained the magnetic meridian for the moment by the collimator, the largest or 

 external iron cylinder. No. 1, was brought in, and placed to the eastward of the com- 



* These ships mount 58 guns each, and have engines of 450 horse power. 



