592 



NATURE 



[August 27, 1908 



other of some i8°. So that the magnetic north is iS" 

 to the west of the true north. 



The angle between the astronomical and magnetic 

 meridian lines is called the magnetic variation, east 

 or west according as the north end of the needle points 

 to the west or east of true — that is, astronomical — 

 norlh at any particular place at any particular time. 



.Such a needle is never at rest, as it is for ever under 

 the influence of the magnetism of the earth, which is 

 always varying. The north point it indicates, there- 

 fore, varies from year to year ; hence the term varia- 

 tion ; it also greatly varies from place to place, so that 

 there is nothing stable about it ; another difficulty is 

 that there may be a local magnetic attraction, caused 

 by iron in the underlying strata, or even gas or water 

 pipes or iron railings, which interferes with the 

 general magnetic attraction at the place, so that a 

 reference to a general chart is insufficient. 



In a survey of any kind, whether of stone monu- 

 ments or houses and trees on an estate, to take 

 instances, the first desideratum is a point of reference 

 to which all measures must be referred ; but the plan 

 as a plan is incomplete unless the relation of the point 

 of reference used to the astronomical north, or the 

 magnetic north, point of the horizon is quite accuratelv 

 shown. 



Xow, the reason that so many archjeologists have 

 dealt with the magnetic meridian and the magnetic 

 north is that it is much more easy to determine it. 

 Unfortunately, it has not struck them that their 

 measures of angles, so far as direction is concerned, 

 are useless unless the relation of the magnetic meridian 

 to the astronomical meridian, at the monument under 

 investigation and at the time of measurement, has 

 been accurately determined. 



It must be confessed that there is much excuse for 

 them, for, until a few years ago, it was difficult in 

 the absence of magnetic surveys to obtain this rela- 

 tion, which consists in an accurate statement of the 

 angle called, as we have seen, the variation between 

 the magnetic and astronomical meridians, or, in other 

 words, the angle between the magnetic and astrono- 

 mical north points of the horizon. 



To give a concrete case of the facts, let us con- 

 sider the case of the Nile Valley, where work such 

 as we are now considering was begun by a Com- 

 mission of the French .\cademv of Sciences in 179S. 



They found that in i7i).S a magnet swung along a 

 line e.xtending from a little to the west of Cairo to 

 the second cataract had a variation of iij° to the 

 west. In i8j4, when the great Lepsius, the prince 

 of archaeological surveyors, arrived on the scene to 

 prepare his majestic plans of the temples, he found 

 the west variation no longer 115°, but 85°. -At the 

 present time the variation is nearer 4° west. But, 

 alas ! in the modern British Schools and Institutes 

 of archaeology little attention is given, to judge from 

 the data shown in the plans they publish, to the ques- 

 tion w'hich we are now considering. .A notable proof 

 of this may be gathered from the fact that, in spite of 

 all the statements and plans that have been made 

 lately concerning the newly explored temple at Deir-el- 

 Bahari, I have been unable to learn whether the indi- 

 cated direction of the axis of the temple is magnetic 

 or true; the onlv information given me, oh! shade of 

 Lepsius! is that the variation had rot been deter- 

 mined bv the surveyors. 



It will be gathered from the above that when we 

 mav have to deal with such a large change of the 

 variation in a centurv. an old nlan with maenetic 

 bearings but without the date of the actual observa- 

 tions is worse than useless. Even when the date is 

 given, a reference to old .Admiralty charts is necessary 

 to get even an approximation to the value of the 

 XO. 2026, VOL. 78] 



variation. This is one objection to the use of the 

 magnetic meridian. 



But, whatever has happened in the past, for the 

 future British archaeologists can hardly be excused 

 from neglecting to compare the magnetic meridian 

 they may use for their plotting with the true or astro- 

 nomical meridian, and stating it on their plans. 



Both the Admiralty and the Ordnance Survey have 

 lately been busily employed in determining the mag- 

 netic variation over the British Isles, and in future it 

 will be shown on every i-inch Ordnance map, so that 

 every archaeologist, for the expenditure of one shilling, 

 will be able to learn the present variation at any 

 monument he may chance to be surveying. Indeed, it 

 may be said that some of the old difficulties are now 

 in a large measure solved. 



The Admiralty have recently prepared a map show- 

 ing this variation for the British Isles for last year, 

 from which archceologists can learn approximately the 

 value of the variation, and hence the direction of the' 

 true north, at any place. 



But because most of the difficulties connected with 

 the observations of magnetic bearings are disappear- 

 ing, it is certain that the magnetic method will still 

 continue to be largely employed, as it is the easier 

 to work with. 



It is not too early to emphasise the important fact 

 that for the astronomical study of the various direc- 

 tions we want, for a reason I shall slate later on, more 

 than the angle from the north point, either magnetic 

 or astronomical, generally termed the azimnlh. We 

 want the angular height of the horizon wliere the lino 

 of direction cuts it. This is called the altitude. 



How .\Z1MLT11 AND .\l.TITfDE .\RE DEFINED AND 

 READ. 



Azimuths. 



The Point Method. — .\ reference to the transactions 

 of antiquarian societies will show that in the past 

 th? most commonly employed method of stating direc- 

 tion, or azimuth, has been by using a compass needle 

 armed with a card such as is used by mariners, and 

 hence called a mariner's compass. This, of course, 

 gives us mag'netic bearings. 



In this the circle is divided into thirty-two parts, 

 called points : four chief magnetic points, N., S., E., 

 \V. ; four quadrantal points, X.E., S.E., S.W., and 

 X.W. ; and twenty-four intermediate points. If we 

 take the N.E. quadrant, for example, the eight de- 

 fining points are N., \. by E., N.N.E., N.E. by N., 

 N.E., N.E. by E., E.N.E.', E. by N. Now as these 

 thirtv-two points cover the 360° in the complete circle, 

 each point contains 11° 15', so that, reckoning direc- 

 tions in this way, there is a play of more than 10° for 

 each statement made. 



But the objection to this method of defining does not 

 end here. If we read the bare statement that a crom- 

 lech, to take .-m instance, is open, say, to the N.E., 

 one is apt to think that the true N.E. is intended; 

 but where the variation is about 22°, as it is now in 

 the west of Ireland, true N.E. is N.N.E. by compass, 

 that is, two points more westerly. 



This svstem of reckoning, then, besides being mis- 

 leading, is too coarse for our purpose, so much so 

 that even mariners are now giving it up, using 

 degrees instead of points. 



The Degree Method. — In the compass card so 

 divided into degrees instead of points we may have 

 o at both the N. and S. point's (mag.), reading to go° 

 at the E. and W. points (mag.), or to 180° at the 

 opposite point. Or, again, we may have 0° at the 

 N. point (mag.), reading through the E., S., and W. 

 points to 360°. Each mag. bearing is now defined 



