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card) 6 degrees to the west of north on true compass (black card). Next set north of deviation compass (green card) 10 degrees to the west of north on red card or Var. compass. Holding the red and green cards in place turn radial arm to SE x E on outside compass, and where the same edge of radial arm cuts the red: card red the correct magnetic course and where it touches the green card read the compass course. As will be seen from the following illustration, which is a pic- ture of the above example on the correc- tor, the correct magnetic course is SE 4 EK and the compass course SE ¥% S. See Fig. 2. Supposing the case of a bearing taken with a compass having a Dev. of 10 de- grees Wly. and a Var. of 6 degrees Wly., and an object bore SE % S and you want- ed to know its true bearing, so that you could lay it off on the chart. Set your Var. and Dev. cards according to your Var. and Dev. and then holding them in place swing radial arm so that its central edge comes in line with SE % S of green card; where the same edge of the radial arm touches the outside compass read off the true bearing and you have it. See Fig. oz, Note.--Be sure to use the correct edge of the radial arm, that is its central edge. It is the edge if continued in a straight line would pass through the center of the cards. Facing the radial arm it is always the left hand edge.. HOW TO SET THE CARDS. Lay the corrector flat on a table or other flat surface and hold it in posi- tion with the left hand. Then with right hand take hold radial arm (down toward its middle or along the circumference of the red card) and revolve it; revolving the radial arm will turn the cards with it. Set the red card to the variation required by means of the radial arm and when set to the required position, hold it down (also with the left hand-- it only requiring a light totich); Set the green card in the same manner--that is, by means of the radial arm. When green card is set as required, hold the green card in position, also with the left hand. Now, set the radial arm to any course required. The cards may also be set without using the radial arm, by simply taking hold of their outer edge with the thumb and first two fingers of the right hand, the thumb on top and the two fingers below. At first the cards and radial arm will work stiff, but a little use will limber them up and make it all the more serviceable. Be careful at first in handling the radial arm, and especially when it works hard, as it will at first. Take hold towards its middle and revolve it stead- ily, not jerkingly, and also in a manner that will not cause it to twist. The end of the radial arm may be bent or bowed "THE Marine REVIEW W- ALSO' TRye., oe RUE COMPASS 29 Co ws, Ta WO '| ie LH ith Me Cen. é o¢ 2 & 20 Tg y >, hoe is. Os "nx Ssvawop ane} FIG. 2:--SHOWING WESTERLY VARIATION AND DEVIATION. upward without harm. By giving the arm a steady pull or push there will be no chance of its twisting. The radial arm is transparent, so that all points and degrees are clearly seen through it. Atter a little practice the operator will get onto his own way of using it, and the above instructions will be unneces- sary. GENERAL RULE FOR THE USE OF CORRECTOR. Set the north point of the red card (with reference to the north point of the black one) in the direction and by the amount of the variation; similarly set the north point of the green card (with reference to the north point of the red one) in the direction and by the amount of the deviation: then any one of the three courses--the true, correct magnetic, or compass course--being given, revolve radial arm till its central edge is upon that course, when the other two are seen on the same edge. To illustrate: suppose a ship heading N x E, deviation 7 degrees Wly., varia- tion 18 degrees Ely.; required the true and correct magnetic courses: set the N-point of red card at 18 degrees east of the N-point of the card in black (true circle), and, while holding it there, set the green circle with its N-point 7 de- grees west of the N-point of the red circle, and holding the two circles with the left hand move the radial arm until its central edge is on N x E of the card in green; then the corresponding divi- sions in line on the same edge will be found to be: on the red card N 4 de- grees E (N % E), being the correct magnetic course; and on the card in black NNE, being the true course. Suppose at the same time, that the ship were in the vicinity of land, where it' was required to fix her position by bear- ings, while heading N x E, and that to plot these on a chart, the corresponding correct magnetic and true bearings were required: all that is necessary is to turn the radial arm to the compass bearing on the green card and the corresponding correct magnetic and true bearings will be seen along the same edge of arm on the red and black cards respectively. One of the best uses the device can be put to is in getting the correct amount of the deviation and how to name it, when your boat is heading on a range. The naming of the deviation is always found the hardest for the beginner. The various rules are easily mixed and mis- takes are the result. Not only this, but! even where one is familiar with the rules, he has no way of proving his work by figures, but by using the corrector the problem is not only solved but proven by a picture of it. If the corrector were of no other value than for the purpose of a proof of one's work, its mission is well filled. It is the proof of one's work that gives him the necessary confidence and self-reliance. How often is the case of the man who has gone along blindly