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244 the meridians shown on a Mercator's projection at a slightly varying angle It is this angle that has to be allowed for in plotting bearings directly on such a chart. Gnomic charts now are pub- lished on which a great circle track 1s represented by a straight line; but these are not in general use. Assume the Cape Race wireless sta- tion is located in latitude 52 degrees, 00 minutes, north, and longitude 50 de- grees, 00 minutes, west, a ship, whose po- sition by dead reckoning is latitude 48 degrees 00 minutes, north, and longitude 38 degrees 00 minutes, west, is given her wireless bearing from the station--what it is, is immaterial for the purpose of this illustration and I have neither charts nor nautical tables at hand. The prob-~ lem is to find the correction to this bearing, so that the navigator may lay his protractor or parallel rulers on the wireless station and determine his line of position. To find this correction the British admiralty gives the following formula: Correction (in minutes) = (% differ- ence in longitude) X sine (middle lati- tude). It may be notcd that some merchant captains use the sine of the latitude on the station, instead of the middle lati- tude between the ship and the station, and claim that their formula gives re- sults of greater accuracy than that of the British admiralty. In the above example the middle lati- tude between the position of the wire- less station and the dead reckoning of the ship would be 50 degrees 00 min- utes, and the difference of longitude would be 12 degrees. Thus, corrections (in minutes) --% X 12-deg. X sin. 50 deg. =360 deg. X sin. 50 deg. =275.8 min. =4 deg. 35.8 min.' This can be worked out also by the traverse table, taking the difference in longitude as distance, the latitude as course, then the departure will be the 'correction. Having the correction of 4 degrees 35.8 minutes the problem now is to determine how it will be applied to the bearing given by the wireless station. A great circle represented on a Mer- cator's chart being a curve concave to the equator, a wireless bearing taken from a station in the northern hemis- phere should be corrected as to make it more southerly before being plotted on the chart. Suppose, Cape Race had informed the ship that her wireless bearing was south 60 degrees 0 minutes east. Then the bearing or rhumb line to be laid off from Cape Race on a Mercator's chart would have been south 60 degrees 0 minutes, east, minus 4 de- MARINE REVIEW . t * "27/7 grees, 36 ininutes, or Say, south 551% degrees east. 'This bearing, plotted on Mercator's projection, would give a line of po- sition which for short distances on either side of the dead reckoning, may be taken as representing the arc of a great circle, on which the ship is located. If a wireless bearing can be obtained from another station and another line of po- sition laid off, the intersection of the two fixes the position of the ship in the same way that cross bearings of visible objects do, though it is subject in a far greater degree to errors. It is, however, a very difficult matte: to plot long distance bearings on @ chart. If you do not believe this take protractors or parallel rulers and try it on a larger chart. Various methods of plotting great circles on a Mercator's chart are available, and it is possible to plot a wireless bearing in the same way, but the results are likely to be dis- appointing. The best plan in dealing with wireless bearings is to figure out the position, where the great circle cuts the dead reckoning latitude or longitude, and start from this, as a basis of the chart work. This can be done by Mercator's sailing, either by computation or inter- polation of the traverse tables, which generally are worked out to distances 9i six hundred miles. Take the wireless bearing as a course and, with either the difference of latitude or longitude be- tween the position of the wireless sta- tion and the dead reckoning position of the ship, it is an easy matter to find the other element you want. If you have not been able by recent observation to determine one or the other element in May, 192) the dead reckoning Position, an adapt tion of the Sumner method can be os ployed to advantage. With the wireless bearing as a course, calculate the lati. tudes for assumed longitudes 30 Minutzs or one degree on each side of the deat reckoning longitude, plot these POsitions on the chart, and draw a line through All the methods of fixing a ship's ne: sition not as valuable, perhaps, as 4 Sumner line of position marked out from the sun, but useful to the extent the wireless bearing can be depended on. All the methods of fixing a ship's po sition by bearings of visible objects car be employed in the case of wireless bearings. As the ship proceeds, she cay receive additional bearings and these worked out and plotted on the chart, with the course and distance and run between them, will enable the navigator tu determine his position with a pre- cision only dependent upon the accuracy oi the wireless bearings. Where the ship is in soundings, a line of po- sition given by a_ wireless pearing may be- of first importance, but con- fidence in fixes determined by di- rectional wireless should never lead the navigator to neglect the lead. Probably more ships 'shave come to grief from failure to make use of the lead than from any other single case. Finishing Propellers With A brasives Abrasives play an important part in the manufacture of marine equip- ment as © they. furnish <a © ready for finishing surfaces _ that under ordinary conditions cannot be means FINISHING THE BLADES OF A BRONZE PROPELLER WITH A FLEXIBLE- SHAFT GRINDER