Elements of electricity, magnetism, and electro-magnetism : embracing the late discoveries and improvements : digested into the form of a treatise, being the second part of a course of natural philosophy : compiled for the use of the students of the University at Cambridge, New England / by John Farrar.

John Farrar

Date:: 1826

Credit: Elements of electricity, magnetism, and electro-magnetism : embracing the late discoveries and improvements : digested into the form of a treatise, being the second part of a course of natural philosophy : compiled for the use of the students of the University at Cambridge, New England / by John Farrar. Source: Wellcome Collection.

Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.

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diminishing the directive force, and removing, as far as possible, all impediments to motion. Thus by reducing the directive force in the ratio of 1 to 0,034, by means of two bar magnets, placed in the line of the dip, Mr Christie found a diurual change in the direction of the horizontal needle, amounting to more than 10®. The dipping needle is likewise subject to daily variations, espe- cially when its directive power is diminished. The following is the result of Mr Barlow's observations relating to this subject. In general, a motion commenced soon after the instrument was adjusted in the morning ; but it was not of that gradual progressive kind which indicated an uniformly increasing or decreasing power, as in the other instrument [horizontal needle.] It passed, for in- stance, suddenly from one half or quarter degree to another, more or less, and which sometimes in the course of the day would give a difference in the dip to the amount of a degree and a half, or evea more ; but I seldom saw in it a tendency to return ; although when I vibrated it toward night, it commonly took up its morning position. I made these observations with the needle in various directions, viz. with the face of the instrument to the east, west, north, south, &c. - but in every case I obtained the same sort of daily motion. The question, therefore, respecting the law of variation of this instru- ment, still remains to be submitted to fixed principles, although there can no longer be any doubt that it is subject to a daily change. —Phil. Trans, for 1823, Part 11. V. Influence of Magnetism on the Rates of Chronometers. Mariner's watches or chronometers, employed to measure time on board of vessels, having in their construction several pieces of steel, some of which are moveable, must evidently be subject te variation in their rate of going, if placed in the vicinity of magnetic bars. This is proved by experiment. Consequently the same effect must take place to a certain degree at sea, both on account of the continual action of the earth, and the magnetic influence of the fer- ruginous masses, by which compass-needles are deflected. For the safety of navigation, it is very important to diminish as much as possible these changes in the rate of going to which chronometers are liable ; and it may undoubtedly be effected, in a great measure, by placing them always in the same place, and as far as possible from compast-needles and magnetic bars, With this precaution;

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