Excerpt from Observations on Terrestrial Magnetism and on the Deviations of the Compasses of the United States Iron Clad Monadnock During Her Cruise From Philadelphia to San Francisco, in 1865 and 1866

The small unmounted magnetic needles were intended to be used for measuring the relative horizontal force on shore and at each of the compasses on board ship. For this purpose it was proposed to vibrate one of them on shore, and then taking it on board ship to the compass at which it was desired to measure the relative horizontal force, to remove the compass card from the centre-point, and putting the small needle in its place, vibrate it again. Unfortunately the small needles were not finished till just before we left Philadelphia, and there was no opportunity of trying them till after we were at sea, when, to my great regret, it was found that the jewels were so small that they would not fit on the centre-point of any compass on board, thus rendering them entirely useless. Under the circumstances, for horizontal force on board ship it was necessary to rely entirely upon measures made with the deflectors belonging to the Admiralty standard compasses - a method certainly not so convenient, and, owing to the constant swinging of the ship when at anchor, probably not so accurate as counting the vibrations of a small needle.

The observations on terrestrial magnetism, and for latitude, time, and true bear ings, were all made by myself and recorded by Mr. Corrin F. Smith, who was cap tain's clerk on the Monadnock, and acted as my assistant when I was observing. My best thanks are due to him for the efficient manner in which he performed his duties, sometimes under circumstances of very considerable physical discomfort.

The reductions and discussions in this report have been made by me, so that I am personally responsible, not only for the general plan of the work, but for every figure contained in it. All the results have been very carefully checked, and it is hoped no material error will be found in them; still, absolute accuracy is scarcely to be expected in any work involvin'g so many figures, the more especially as much of it has been done during moments snatched from other and more pressing professional duties.

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