EXAMPLES FOR PRACTICE. 1. 1885.-January 21st. The observed meridian altitude of the star "a Pegasi" bearing South was 59° 10′ 30′′. Height of eye 17 feet. Required the latitude. 1885.-February 16th The observed meridian 21 66 altitude of the star Regulus" bearing North was 45° 3′ 0′′. Height of eye 18 feet. Required the latitude. 3. 1885.-March 18th. Height of eye 20 feet. "a Orionis" bearing South was 57° o' o". 4. 1885.-April 15th. The observed meridian altitude of the star" Achernar" 5. 1885.-May 1st. The observed bearing South was 78° 59′ 50′′. latitude. 6. 1885.-June 24th. The observed bearing South was 42° 10' 0". latitude. Required the 7. 1885.-July 11th. The observed meridian altitude of the star "a Scorpii" bearing South was 64° 19′ 0′′. Height of eye 17 feet. Required the latitude. 8. 1885.-August 26th. The observed meridian altitude of the star "Vega" bearing North was 50° o' o". Height of eye 18 feet. Required the latitude. 9. 1885.-September 27th. The observed meridian altitude of the star "Canopus" bearing South was 52° 37′ 50′′. Height of eye 19 feet. Required the latitude. 10. II. 12. altitude of the star 1885.-October 10th. The observed meridian 1885.-November 7th. The observed meridian altitude of the star "Sirius" bearing North was 69° 4' 0". Height of eye 21 feet. Required the latitude. 1885. December 20th, The observed meridian altitude of the star "Aldebaran" bearing North was 51° 3′ 40′′. Height of eye 20 feet. Required the latitude. NAPIER'S DIAGRAM. THE USE OF THE DIAGRAM.- By this most useful and ingenious diagram, the deviation of the compass for all directions of the ship's head can be found sufficiently near for practical purposes by taking the bearing of a distant object when the ship's head is at any eight equidistant points. The eight points given at the examination are the four cardinal points N., S., E., and W., and the four quadrantal points N.E., S.E., S.W., and N.W. TO FIND THE CORRECT MAGNETIC BEARING.-The mean of the eight given bearings is considered as the correct magnetic bearing of the distant object, that is the bearing of the object from the ship by a compass unaffected by any iron. To find the mean of the bearings we must consider :First.—If all the bearings are named alike. Second. If some are named N. and some S. And Third.-If some are named N. and some W. First Case. All bearings named alike.-Add all the bearings together, divide the sum by 8, and name the result the same as the bearings. Second Case.-Some bearings N. and some S. Either take each of the N. bearings from 180°, or each of the S. bearings from 180° (whichever is the less trouble). Then add all the bearings together, and divide by 8, naming the result the same name as the unaltered bearings. Third Case. Some bearings E. and some W. First add together all E. bearings. Then add together all W. bearings. Subtract the less sum from the greater, divide remainder by 8, naming the result the same name as the greater. The result in each of the above cases is the correct magnetic bearing. TO FIND THE DEVIATION.—In the First and Second cases the difference between the correct magnetic bearings and each of the bearings by the standard compass is the deviation for each position of the ship's head, In the Third case, when the correct magnetic bearing and the bearing by the standard compass are both E. or both W., subtract the less from the greater. But if one is E. and the other W., then add them together for the deviation. To name the DEVIATION.-When the correct magnetic bearing is to the right hand of the bearing by the standard compass the deviation is E., and when to the left hand it is W. Note. It will be useful to notice that in finding the deviation the correct result will be obtained by considering the correct magnetic bearing and the observed bearing as two azimuths, and proceed as in finding the error of the compass. Place one TO DRAW THE DEVIATION CURVE. With a pair of dividers take from the centre line of Napier's diagram the number of degrees in the deviation for N. leg of the dividers on N., and lay the other on the DOTTED line passing through N., to the right if the deviation is E., to the left if the deviation is W., and with a pencil make a distinct dot where the dividers reach. Proceed in a similar manner with each of the other deviations, making a dot each time. After all eight are laid off, draw a flowing line passing through all these dots; this line is the deviation curve. Given a correct TO USE THE DEVIATION CURVE.-Ist. magnetic course, to find the corresponding compass course. This is Question No. 8. Proceed thus :-Place one leg of the dividers on the given course on the centre line, extend the other leg to the curve on or parallel to a PLAIN line. Keep the leg that is on the curve still, and move the other leg until it cuts the centre line in another place. The number of degrees from N. or from S. there indicated is the compass or steering course, which must be named N. or S. towards the E. or W., and be placed below the given correct magnetic course. F 2nd. Given a course which has been steered by compass to find the correct magnetic course made. This is Question No. 9.-Proceed thus: Place one leg of the dividers on the given course on the centre line, extend the other leg to the curve on or parallel to a DOTTED line. The leg that is on the curve must be kept still, but move the other until it cuts the centre line in another place. The number of degrees from N. or from S. there indicated is the correct magnetic course, which must be named N. or S. towards the E. or W., and be placed below the given compass course. 3rd. Given the direction of the ship's head, and the compass bearings of two distant objects, to find the correct magnetic bearings of those objects.-This is Question No. 10. Proceed thus-Place one leg of the dividers on the direction of the ship's head on the centre line, and extend the other leg to the curve on or parallel to a DOTTED line. Then keeping the legs the same distance apart, lift the dividers from the paper, place one leg on the first given bearing on the centre line, and lay the other on the centre line : Upwards if deviation for ship's head is W. Downwards if deviation for ship's head is E. The number of degrees there indicated will be the correct magnetic bearing from N. or from S. towards the E. or W., which bearing must be placed below the first given compass bearing. Proceed in the same manner with the other bearing. This completes Questions 7, 8, 9, and 10. |