C. GENERAL PHYSICS. 157 



fessor Harkness, of the Naval Observatory, containing the re- 

 sults of an investigation made by him into the magnetic con- 

 dition of the Miantonomah, a study that was prosecuted dur- 

 ing the entire period of the voyage of that monitor from New 

 York, around Cape Horn, to San Francisco. In order to test 

 all the compasses used by the navy, a compass observatory 

 has been erected at Brookline, Mass. ; the building being con- 

 structed at the expense of Mr. Ritchie, while the Bureau of 

 Navigation lends the requisite instruments. The building 

 thus provided is situated at least one hundred feet distant 

 from any other structure, and has been carefully and thor- 

 oughly built, without the use of magnetic material in any 

 part of it. The whole arrangement is essentially the same 

 as that used for the examination of the compasses of the Brit- 

 ish Admiralty. An important part of the duties assigned to 

 Professor Greene has reference to the determination of the 

 deviation of the compass, and the modifications of its mag- 

 netic force, that are due to the action of the ship's iron. For 

 several years past the ships of war have generally been 

 " swung " before going to sea, for the determination of the 

 compass deviations. Nearly all our sea-going ships have 

 wooden hulls, and the magnetic perturbations are, therefore, 

 of but slight account; but now it is proposed to extend these 

 observations to all iron-built ships, whence the requisite data 

 will soon be acquired for investigating the peculiarities of these 

 vessels, and the variations in the same depending upon the 

 movement of the ship from one portion of the globe to the other. 

 In June last Professor Greene examined, on the Northern 

 lakes, the United States steamer Michigan, which is the oldest 

 iron-built ship in the navy. With reference to this vessel, he 

 concludes that, in general, there is a marked difference be- 

 tween the variations of the fore-compass and the after-com- 

 pass, being in one of the compasses a minimum when the ves- 

 sel heads northerly, and a maximum when it heads southerly, 

 while they are the reverse at the other compass. The polar 

 magnetism of the ship* is very nearly symmetrical with the 

 midship or fore and aft section. The influence of the iron 

 steam-cutter is unimportant. The original polar magnetism 

 of the Michigan, as developed while she was being built, can 

 not be very exactly investigated ; but he concludes that, in 

 general, the original direction of the permanent magnetism 



