November 21, 1901] 



NA TURE 



57 



mentofthe avenue, and four others at distances averaging 

 100 feet apart nearer the further recognisable ex- 

 tremity, and their directions were measured with the 

 theodoHte, independently by two observers, the reference 

 point being Salisbury Spire, of which the exact bearing 

 from the centre of the temple had been kindly supplied 

 by Colonel Johnston, R.E., the Director-General of 

 the Ordnance Survey. The same was also measured 

 locally by observations of the sun and of Polaris, the 

 mean of which differed by less than 20" from the Ord- 

 nance value. The resulting observations gave for the 

 axis of the avenue nearest the commencementan azimuth o 

 49 38' 48", and for that of the more distant part 49^32' 54". 

 The mean of these two lines drawn from the central 

 interval of the great trilithon, already referred to, passes 

 between two of the sarsens of the exterior circle,which have 

 an opening of about 4 feet, within a few inches of their 

 middle point, the deviation being northwards. This may 

 be considered to prove the close coincidence of the 

 original axis of the temple with the direction of the 

 avenue. 



This value of tht azimuth, the mean of which is 

 49" 35' 51", is confirmed by the information also supplied 

 from the Ordnance Office that the bearing of the 

 principal bench mark on the ancient fortified hill, about 

 eight miles distant, a well-known British encampment 

 named Silbury or Sidbury, from the centre of the temple 

 is 49' 34' 18", and that the same line continued through 

 Stonehenge to the south-west strikes another ancient 

 fortification, namely Grovely Castle, about six miles dis- 

 tant and at practically the same azimuth, viz. 49' 35' 51". 

 For the above reasons 49" 34' 18" has been adopted for 

 the azimuth of the avenue. 



The present solstitial sunrise was also watched for on 

 five successive mornings, viz. June 21 to 25, and was 

 successfully observed on the latter occasion. As soon as 

 the sun's limb was sufficiently above the horizon for its 

 bisection to be well measured, it was found to be 8' 40" 

 northwards of the peak of the Friar's Heel, which was used 

 as the reference point, the altitude of the horizon being 

 35' 48". The azimuth of this peak from the point of 

 observation had been previously ascertained to be 

 50' 39' 5 "1 giving for that of the sun when measured, 

 50' 30' 25", and by calculation that of the sun with the 

 limb 2' above the horizon should be 50^ 30' 54". This 

 observation was therefore completely in accordance with 

 the results which had been obtained otherwise. 



The time which would elapse between geometrical 

 sunrise, that is, with the upper limb tangential with the 

 horizon, and that which is here supposed, would occupy 

 about seventeen seconds, and the difference of azimuth 

 would be 3' 15". 



The remaining point is to find out what value should 

 be given to the sun's declination when it appeared 

 showing itself 2' above the horizon, the azimuth being 

 49' 34' 18". 



The data thus obtained for the derivation of the 

 required epoch are these : — 



(n The elevation of the local horizon at the sunrise 

 point seen by a man standing between the uprights of 

 the great trilithon (a distance of about 8000 feet) is 

 about 35' 30", and 2' additional for sun's upper limb makes 



37' 30"- 



(2) —Refraction -t- parallax, 27' 20". 



(3) Sun's semidiameter, allowance being made for 

 greater eccentricity than at present, 15' 45''. 



(4) Sun's azimuth, 49 34' 18", and N. latitude, 51' io'42". 

 From the above data the sun's declination works out 



23' 54' 30" N., and by Stockwell's tables of the obliquity, 

 which are based upon modern determinations of the 

 elements of the solar system,' the date becomes 1680 B.C. 



1 " Smithsonian Conlributions to Knowledge," vol. -xviii. No. 232. 

 Table ix. (Washington, 1673.) 



NO. 1673, VOL. 65] 



It is to be understood that on account of the slight un- 

 certainty as to the original line of observation and the 

 very slow rate of change in the obliquity of the ecliptic, 

 the date thus derived may possibly be in error by ±200 

 years. 



In this investigation the so-called Friar's Heel has 

 been used only as a convenient point for reference and 

 verification in measurement, and no theory has been 

 formed as to its purpose. It is placed at some distance, 

 as before-mentioned, to the south of the axis of the 

 avenue, so that at the date arrived at for the erection of 

 the temple the sun must have completely risen before it 

 wasvertically over the summit of the stone. It may 

 be remarked further that more than 500 years must 

 yet elapse before such a coincidence can take place at the 

 beginning of sunrise. 



We have to express our thanks to Sir Edmund 

 Antrobus, Bart., for much kind assistance during our 

 survey ; and to Mr. A. Fowler and Mr. Howard Payn, for 

 skilful and zealous cooperation in the measurements and 

 calculations. As already stated. Colonel Duncan A. 

 Johnston, R.E., Director-General of the Ordnance 

 Survey, has also been good enough to furnish us with 

 much valuable information, for which our best thanks are 

 due. 



TWO BOOKS OF TRAVEL."^ 



IN its general scope and character the first-named of 

 these two works is very similar to Hudson's "The 

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 ledge. So far as the title is concerned, it might well be 

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 during recent travel, but this is far from bemg the case, 

 the author's journeys having been made during the 

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To judge from its title (in the selection of which we 

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1 "The Great Deserts and Forests of North .\merica." By Paul 

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" Sunshine and Surf; a Year's Wanderings in the South Seas," By 

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