MAGNETIC OBSERVATIONS. 121 



the cover of the compass, as the ship's head pointed successively to each of the 

 true magnetic points, was computed by means of the formula 



R = 180° -J- A — ? 

 where 



jR = reading of sight vanes on the azimuth circle attached to the cover of the 



• compass. 

 A = true magnetic azimuth of the distant object ; the azimuth being counted 



from the south around by the west. 

 £ == azimuth of the ship's head, counted from the correct magnetic north around 

 by the east. 



This having been done, on a tolerably calm day steam was got up in the boilers, 

 and, the vessel riding at a single anchor, slack water was waited for. As soon as 

 the tide ceased to run, the executive officer took the deck ; an officer was stationed 

 at each of the compasses; I' went to the Admiralty Standard; and a quartermaster 

 was stationed at the ship's bell. Then the helm was put hard-a-starboard, or hard- 

 a-port, depending on the direction in which it was desired to have her head swing, 

 and the engines having been started, one forward and the other backward (the 

 Monadnock was provided with twin screws which were entirely independent of 

 each other), the vessel at once began to turn, without bringing any considerable 

 strain on her cable. Her motion was perfectly under control, and could be made 

 fast or slow at pleasure by merely varying the speed of the engines. I then set 

 the sight vanes of the Admiralty Standard Compass to the reading (on the azimuth 

 circle) of the point at which the ship's head would first arrive, and placing my eye 

 to them I watched for the instant when they pointed to the distant object chosen 

 as an azimuth mark. As the thread of the sight vane approached the object I 

 cautioned the quartermaster to be ready, and at the instant it covered the object I 

 made a signal, by dropping my outstretched arm, and tbe quartermaster struck a 

 single stroke on the bell. Upon hearing this, every officer at once read off and 

 recorded the heading of the ship, as indicated by the compass at which he was 

 stationed. Then, the engines not having been stopped, I turned the sight vanes 

 forward to the reading of the next point, and the same process was repeated ; and 

 so on, till the readings of all the compasses had been observed at each of the 

 thirty-two points, which was generally accomplished in about an hour, or an hour 

 and a half. The difference between any observed reading and the true point to 

 which the vessel's head was directed at the time that reading was made, was of 

 course the deviation of the compass on that point. 



The forward iron and wooden pilot-houses were fixed and did not revolve with 

 the turret, so that the lubber lines of the compasses in them always remained in 

 the same position. But with the after iron and wooden pilot-houses the case was 

 different. They were attached to the turret and revolved with it, and by so doing 

 caused the lubber lines of the compasses in them also to revolve. As the turrets 

 were frequently turned, it became necessary to establish marks by which the 

 position of the after one could always be referred to some fixed position, so that a 

 correction could be applied to the readings of the compasses in its pilot-houses to 



16 August, 1872. 



