September 24, 190SJ 



NA TURE 



511 



miralty. The full smoothed line shows the average 

 position of the line of equal variation for 1907, the 

 iJotted line the variation obtained from land observa- 



FlG. 



-The prismatic compass, showing the sighting arrangement and 

 manner of use. 



the slit, SO that this, the wire, and the object 

 observed are seen together. The graduation runs 

 from 0° to 360°, the zero lying in the N. point of the 

 magnetic meridian, so that the graduation read is the 

 magnetic azimuth of tlie object seen through the slit 

 in line with the wire. 



In order to get a zero reading under the prism 

 when we are looking magnetic north, the zero of 

 giaduation is at the magnetic south end of the needle. 



The support of the fine wire also carries a hinged 

 mirror, by means of which the azimuths of objects 

 considerably above the horizontal line can be 

 measured. For measuring the azimuth of the sun 

 on the horizon, dark glasses are attached to the slit 

 plate, which can be thrown into use when required. 



When at work, when the box is rotated to bring 

 any object in the line of the slit and wire produced, 

 the needle, and with it the graduated ring, remains 

 steady. 



An ordinary level, furnished with a needle and card, 

 can also be used for taking magnetic azimuths alone. 



The 25-inch maps of the Ordnance Survey have put 

 into the hands of archseologists a tremendous engine 

 of research, from which true azimuths can be at 

 once found without the intervention of a magnetic in- 

 strument in the field. To do this a circitlar protractor 

 is employed in the manner shown in the accompanying 

 figure. 



In the case of the Avenue represented, a line some 

 \2 inches long is drawn parallel to its length. 

 .Another line is then drawn parallel to the side of the 

 map, which is always a N. and .S. line, or very nearly 

 so. The zero of the protractor is brought on the N. 



tions alone, and the dot and circle 

 line that got by observations at 

 sea alone. 



It will be seen that there is a 

 strange divergence between the 

 land and sea observations, but in 

 spite of this the chart enables us 

 to estimate the variation at any 

 place on it within half a degree 

 without astronomical observation. 



I am glad to learn that the use 

 of the mariner's compass pure and 

 simple is now rapidl}- going out of 

 use so far as archasologists are 

 concerned, and for the rapid 

 measurements of azimuths alone, 

 using magnetic bearings, the azi- 

 muth, or prismatic, compass is 

 the instrument generally emploved. 



It is cheap, light and handy. 

 In the smaller instruments the 

 needle is attached to the under sur- 

 face of a compass card showing 

 the thirty-two magnetic points. 

 In the best forms a magnetised 

 bar having an agate centre 

 balanced on a steel pivot carries 

 an aluminium or silver ring, which 

 is graduated to half degrees, and 

 with many monuments a greater 

 accuracy than this is not possible. 

 Its general arrangement will be 

 gathered from Fig. 11. At one 

 end of the bo.\ is a fine wire, at 

 the other a right-angled prism ; 



above the prism is a narrow slit, through which the 

 wire is observed over the centre of the graduated ring. 

 The prism reflects to the eye the graduation under 



VOL. 7S] 



Fig. 12. — The circular protractor measuring the azimuths of an avenue on a 25-inch Ordnance Map 



of Dartmoor. 



and S. line, and the centre on the point of intersec- 

 tion. The angle between the two lines is the azimuth. 



NOR.MAN LOCKVER. 



