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as for example the iron oarlocks. Two wooden outriggers are constructed from 4- by 

 4-inch by 16-foot lumber and installed temporarily athwartship on the gunwales, one 

 forward and one aft. To the four ends of the outriggers are hung four canvas sea 

 anchors, each approximately 36 mches in diameter. The sea anchors should be sus- 

 pended well below the surface of the water, so they will be beneath any wave 

 motion, and be weighted by 16-pound sounding leads to ensure that the lines supporting 

 them will be taut at all times. These sea anchors are for the purpose of minimizing 

 any rolling motion of the boat. 



The observations may be made on a ship standard magnetic compass, but a special 

 spherical-type compass is preferable if available. The spherical glass top magnifies 

 the compass card, which is equipped with a shadow pin of sufficient height to permit 

 the magnetic bearings of the sun to be read directly, provided its altitude is not too 

 great. The compass should be mounted on a stand in the center of the boat. No 

 iron nails, bolts, or other magnetic metal should be within 4 feet of the compass. 



The same method is used in swinging a small boat as in swinging ship (see 4415). 

 The man at the steering oar stands where he can see the compass and steadies the boat 

 on each desired heading. A swing to the right on 24 headings is immediately followed 

 by a similar swing to the left on the same headings, four observations on the sun being 

 taken on each heading. 



This method of making magnetic observations should be attempted only when 

 the sea is comparatively calm. When the survey ship is engaged on offshore sounding 

 lines it is frequently practicable to lower a boat party to make the observations, pick- 

 ing it up again on a return sounding line, with very little delay to the progress of the 

 work. A complete set of such observations can be made in about 2 hours. 



Azimuths may be obtained by observations from the boat, on the sun or on a 

 distant land object whose position is accurately known, or from land by an observer 

 at a known position. In the second case the position of the vessel or boat should be 

 fixed at the time of each observation. 



154. Survey Numbers 



1541. Field Numbers 



For convenient reference during the season, each hydrographic sheet shall be 

 assigned a field number. This field reference number shall be composed of the first 

 two letters of the name of the survey vessel, followed by a 4- or 5-digit number, the last 

 two digits of which represent the year, the third from the last representing the number 

 of the sheet in the series, and the first one or two numbers representing the scale, 

 any scale larger than 1 : 10,000 being indicated by a zero as the first digit. For example: 

 Oc-8341 would designate the third 1: 80,000 scale sheet initiated by the ship Oceanogra- 

 pher in 1941. The final two digits represent the calendar year in which the survey was 

 initiated, or the number was assigned, and are not to be changed if the survey extends 

 into the following calendar year. 



For the same use each topographic sheet shall be designated by a capital letter 

 assigned m alphabetical order during the season, a new series starting with "A" each 

 season. The complete reference shall be composed of the first two letters of the name 

 of the survey vessel, followed by the capital letter, followed by the last two digits of 

 the year, all connected by hyphens. For example: Ex-B-41 would indicate the second 

 topographic sheet initiated by the ship Explorer in 1941. 



