PART II. DEVIATION OF THE COMPASS. CHAPTER I. Errors of the Compass-Variation-Deviation-Local Attraction-How a Table of Deviations is formedBy Reciprocal Bearings-By the Bearings of a Distant Object-By Marks on Dock Walls-By Azimuth and Amplitude Observations-How a Deviation Table is used-Examples CHAPTER II. Composition of Forces-How Compasses act under the CHAPTER III. On the Co-Efficients A B C D E-Semicircular Deviation produces Co-Efficients в and c-Quadrantal Deviation produces and E-A is constant-How the CoEfficients are Found-Example of finding the CoEfficients. PAGE 37 48 57 CHAPTER IV. On Heeling Errors-How the Amount of Heel is Ascer- 77 CHAPTER V. Compensations-Definition-Comparison of Compensated and Uncompensated Compasses-Dumb CardCompensation of Semicircular Deviation-Compensation of Quadrantal Deviation-Compensation for Heeling Error. PAGE 85 CHAPTER VI. Napier's Graphic Method-How the Diagram is Constructed-Explanation of the Diagram-How the Diagram is used-Examples . EXAMINATIONS OF MASTERS AND MATES. Notice of Alteration in Examination Papers. NOTICE OF EXAMINATIONS. Compass Deviation 93 110 . 117 PART I. MAGNETISM. CHAPTER I. MAGNETIC FIELD DEFINITION-POLES, MAGNETIC EQUATOR AND AXIS- SUBSTANCES. 6 THEORY OF MAGNETIC 1. Definition.- Magnetism is that branch of physics which treats of the properties of magnets and of their action on each other.' The term is also sometimes applied to the force which produces the effects called magnetic. Magnets are of two kinds, natural and artificial. A natural magnet, sometimes called a loadstone, is the ore of iron known as the magnetic oxide, and its composition is represented by the chemical formula Fe, 04. This ore has the power of attracting pieces of iron and steel, and of conferring its own properties on these bodies. The loadstone is found in Scandinavia, Elba, and in many parts of the United States, especially in Arkansas. The Magnet derives its name from Magnesia in Lydia (Asia Minor), where its properties were first noticed. Artificial magnets are straight or bent bars of iron or steel which have acquired magnetic properties. Several such B bound together form a magnetic battery, whose power is much less than the sum of its components. In experiments artificial magnets are chiefly used because they can be more easily obtained, and can be made of any shape and of great strength. 2. Poles, Magnetic Equator and Axis.-If a magnet, either natural or artificial, be rolled in iron filings and then withdrawn, we find that the filings are accumulated most abundantly at two nearly opposite points. These points are the poles of the magnet, and are the centres of greatest attraction and repulsion. Between these two points a line is found where the filings do not adhere: this is the Neutral line or Magnetic Equator. Every magnet has two poles and a neutral line; but often, from want of care in magnetising bars of steel, more than two poles, lying between the extremities, are produced: these are called Consequent points or poles. The straight line joining the magnetic poles is called the magnetic axis: and if the body be symmetrical and homogeneous this axis is the axis of the figure. When either of the poles of a magnet is held at a short distance from pieces of iron or steel, they are attracted to it and adhere with considerable force. If a steel needle be suspended by a fine thread and brought near to a pole of a magnet, it will be attracted; and if we place the needle through a small piece of cork and allow it to float on the surface of a tumbler of water, and then bring the pole of a magnet outside the tumbler, the needle will be attracted towards the magnet, and the force with which it is attracted is not diminished from the interposition of glass. It is found that magnetic force acts through almost every substance except iron in several of its combinations, and the other elements susceptible |