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42 the. terrestrial magnetism influenced it. Now, carry the suspended needle a little beyond the end of the bar mag- net (toward the true pole as it were), and it will be found to turn complete- ly round (180°) and point in the re- verse direction, that is, that end of the needle points constantly to that pole of the magnet, and will continue to so long as it is held within the in- fluence of the bar magnet. This is the precise manner in which the earth's magnetism operates on the compass used for navigating purposes. The variation of the compass, then, when the observer is directly between 'the magnetic and true poles, as in the above example, would be 180°, the greatest. amount of variation possible. Captain Ross discovered the ap- proximate positions of the magnetic poles by means of the dipping needle, and his experience was identical with the above experiment. To reach the trite north pole'of the earth, if it were possible, from the magnetic north pole we would have to steer due south by compass--from the south magnetic pole to the true south pole due north by compass. THE ABSURDITY OF ATTEMPTING TO INSU- _ LATE THE COMPASS FROM THE "MAGNETISM OF THE SHIP. Towson says that in connection with compass. deviations, many practical men have vainly attempted to discov- er some substance or medium that would insulate the needle from the in- fluence of the magnetism of the ship's iron.. Many imagined discoveries of this character have been patented, and have served both to waste the time and money of .the patentees and to distract the attention of the mariner from that class of study which alone can promote his safety in navigating his ship. It may be stated with con- fidence that there is no available me- dium that can intercept magnet in- fluence. For two centuries, at least, every class of bodies has been sub- mitted to experiment, in order to dis- cover a material capable of intercept- ing the influence of one magnet on the other, not for the purpose of pre- venting deviation, but because mechanic clearly perceives that if such a material were discovered a motive power could be produced by various arrangements of permanent magnets and insulating bodies. But no one has succeeded in making the discovery. Should, however, the efforts, which for two centuries have been unsuc- cessful, be realized, although 'a new motive would be available, it would be altogether valueless in connection with compasses of ships. The mag- the - THE Marine ReEvIEW netism of the earth generally, the » loadstone, soft iron, hard steel, or the electro-magnet, is of the same nature. If we shut off one we shut off all. If, therefore, we could succeed in insulating the needle from the magne- tism of the ship we could by the same means intercept the magnetism of the earth, and thus the compass would be rendered absolutely useless. In the first place, then, the object sought for is not obtainable; and, secondly, if such a medium did exist, it would be entirely valueless in connection with the compasses of ships. HOW TO TELL A GOOD COMPASS. A simple test for a compass is to place it in position, noting the read- ing thereof, and with a pocket-knife, or piece of steel, deflect the needle, remove the attraction quickly, and the time it takes to come to rest at the exact point it started from. If the card is very slow in coming to rest, or does not return to the exact point - it started from, something is wrong with the compass. Either the mag- nets are weak, the weight too great -- on the point of support, causing fric- tion, or the jewel cap may be defec- tive. In purchasing a compass always give it this test. Remember, the com- pass that will show the quickest num- ber of vibrations in a given number of seconds is the best instrument for any and all purposes. INVENTORY OF THE COMPASS. In the navy, nished for recording in detail all the information in regard. to sensibility, directive force and general condition © of each compass on board. Instructions on the back of . each form or report, explain the method of making the necessary observations, and require that the form shall be filled out and sent to the bureau of equipment in January of each year. The test for sensibility is to take the reading of the compass before dis- turbed, and again after it comes to rest. If there be any difference in the two readings it shows the sensibility to be poor. The card is deflected to the right and to the left of the point of rest, The test for directive force is simi- lar to the test for sensibility. This is performed by the method of time vi- brations; that is, the reading of the compass is noted when at rest, and then made to vibrate to the right and to the left, and the difference in time for the same number of vibrations should correspond. The difference in the time for the same number of vi- brations shows weak directive force. The time is marked to the nearest blank forms are fur-. fractional part of a second required by the compass card to make ten com- plete vibrations across its starting point. In counting the vibrations it must be observed that the time 'is marked at the first passing of the recxoning point on the card, at the next count "one" at the next "two" and so on to "ten.'. Mark the time at "ten." The interval-of time be- tween "mark" and "ten' will be the time by the needle to make ten vibra- tions. For example, if it takes 11 sec- onds for the card to make 10 com- plete vibrations to the right, it should also take 11 seconds to make 10 vi- brations with a swing to the left. If not it shows weak directive force. The -amplitude of arc by which the needle is drawn aside should be as small as possible consistent with ac- curately noting the tenth vibration across the zero, or. starting point, which should allow the needle to pass two or three divisions beyond, in or- der to be sharply defined. The same amplitude should be used for each set of vibrations. QUESTIONS FOR MASTERS AND MATES.--NO. 13. 190. What kind of liquid in a Baker compass ? 191. What is the system of reading the compass by degrees? 192. What is the meridian of. compass? Why? 193. How-does NE by E derive its name? 194. Your compass course is NE with the wind north, and you are making %4- pt. leeway. What course should you steer to. counteract the effects of the leeway? : 195. Owing to stress of weather you are. forced to run into the wind and sea. Your course by compass is WNW, and you have run on this course for 16 hours, making 7 miles per hour; the Var. at start is 214° Ely. and at end 5° Ely, and the Dev. for WNW is %-pt. Ely, what is the true course you have made good and how would you find your po- sition on the chart? '196. You have an old chart dated 1898, which gives the Var. at a certain. place as being 3° 15' Wly, and the an- nual change is 5', what is the Var. for 1906? 197. Heading on Frying Pan and Pipe Islands range your compass says N5SZE, how much and which way is the deviation? 198. When a patent log _ indicates more miles than the ship has run, what might be the trouble with it? 199. What influences affect the mostly? 200. 'With a following sea will your the log