MAGNETIC OBSERVATIONS. 119 



SECTION V. 



OBSERVATIONS ON THE MAGNETISM OF THE SHIP. 



The Monadnock is a second rate iron-clad vessel, of the Monitor type, of 1564 

 tons old or 1091 tons new measurement. On deck her length is 260.5 feet, and 

 her breadth 52.0 feet. She has a wooden hull, but her deck is covered by three 

 layers of iron plates, each one inch thick; and her sides, for a depth of five feet 

 from the deck, are covered by six layers of iron plates, each one inch thick. Thus 

 the deck is protected by three, and the sides by six inches of iron. She is provided 

 with two iron turrets, cylindrical in form, each 22.8 feet in outside diameter, 9.0 

 feet high, and 1 1 inches thick. On top of each of them stands an iron pilot-house, 

 7.7 feet in outside diameter, 6.4 feet high, and 11 inches thick. Each of these 

 pilot-houses is cylindrical in form, and so placed that its axis coincides with the 

 axis of the turret upon which it stands. The sides of the turrets and pilot-houses 

 are not solid, but are composed of iron plates, each one inch thick, placed one 

 upon the other and bolted together till a total thickness of eleven inches is attained. 

 To each of the iron pilot-houses arc bolted wooden stanchions, which carry wooden 

 pilot-houses whose floors are about nine and a half feet above the tops of the iron 

 pilot-houses. The centres of the wooden pilot-houses arc respectively in the same 

 vertical lines with the centres of the turrets and iron pilot-houses over which they 

 stand. The centres of the turrets coincide with the midships line. The distance 

 from the stern of the vessel to the centre of the after turret is 84.5 feet; from the 

 centre of the after turret to the centre of the forward turret, 99.1; and from the 

 centre of the forward turret to the cut-water, 76.9 feet. Passing forward from the 

 after turret, we come first to the ventilator, which is 6.5 feet in diameter, and 22.8 

 feet high above the deck; and then to the smoke-stack, which is 9.9 feet in dia- 

 meter, and 31.0 feet high above the deck, both it and the ventilator being of iron. 

 The distance from the centre of the after turret to the centre of the ventilator is 

 31.3 feet; from the centre of the ventilator to the centre of the smoke-stack, 16.5 

 feet ; and from the centre of the smoke-stack to the centre of the forward turret, 

 51.3 feet. 



At St. Thomas, before the magnetic observations on board ship were made at that 

 place, a wooden mast 77.7 feet high was placed on the ship in order to enable her 

 to carry some sail. Its centre is 22 feet forward of the centre? of the forward turret, 

 and what little iron was used in its construction is so placed that it is not at all 

 probable that it affected the deviation of the compasses in its neighborhood in the 

 slightest. 



