NATURE 



[November 2, 191 1 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature, 

 No notice is taken of anonymous communications.] 



Irregular Long-period Changes in Level. 



At the Portsmouth meeting of the British Association 

 Mr. V. Napier Denison contributed a paper on changes in 

 level observed with a horizontal pendulum at Victoria, 

 B.C. Observations commenced in January, 1899, and ex- 

 tended over the next eleven yea!^. I'or the most part these 

 refer to changes in level in an • asi-wcst direction. Like 

 observers in other parts of the world, he found diurnal, 

 annual, and other changes, all of which can be referred to 

 epigenic influences. 



Over and above these Mr. Denison pointed out that the 

 pendulum did not annually return to its normal position ; 

 for irregular periods varying between twelve and thirty 

 months the zero might travel eastwards, after which it 

 would make a greater or less excursion towards the west. 

 But here comes the interesting point. A curve of these 

 wanderings very closely agrees with one representing the 

 innual frequency of world-shaking earthquakes, which 



have been most numerous when the pendulum was farthest 

 removed from its normal position, whether this was to the 

 east or west. The accompanying curves by Mr. Denison 

 are self-explanatory. 



With the object of throwing further light on these 

 observations it would be of interest to learn whether these 

 long-period changes in level, and, I may add, in azimuth, 

 have been recorded at observatories which have piers on 

 rock foundations, particularly in districts where there is 

 reason to suppose rock folding may still be in progress. 

 Such observatories may perhaps be found in Switzerland, 

 Italy, the Balkans, North India, and the American and 

 Asiatic shores of the Pacific. I have not, however, been 

 able to find a catalogue which gives information about the 

 foundations of astronomical observatories. 



An excellent risumi relating to changes in the vertical 

 is given by Sir G. H. Darwin in a report to the British 

 Association (1882). What is now required is a rdsumd 

 since 1899, from which date we possess a fairly complete 

 catalogue of world-shaking earthquakes, each of which, 

 there is reason to suppose, may be regarded as the 

 announcement of a general relief in seismic strain, and as 

 one earthquake may beget another, they frequently take 

 place in widely removed districts at about the same time. 

 If a megaseism means a relief of strain in the crust of 

 our world, can astronomers throw anv light upon its 

 growth ? 



NO. 2192, VOL. 88] 



All who are interested in earth physics would like to 

 know whether evidences of long-period changes in th' 

 vertical exist, particularly in the direction of the dip »<! 

 strata on which their observatories are situated. 



Shide, Isle of Wight, England. Joh.n Milm.. 



Solar Eclipse— April, 1912. 



A FEW years ago I read a short paper before the British 

 Astronomical Association, in which I suggested that certai: 

 eclipse observations should be made, not at the centre lii. 

 of the path of the shadow, but as near as possible to it 

 two edges; and seeing that the forthcoming eclipse is ui 

 suitable, on account of its short duration, for the usual 

 observations, I hope that the astronomers of Europe will 

 devote their attention to determining the position and 

 width of the shadow. If they can do this with the exacti- 

 tude which I anticipate, then, in a single day, full informa- 

 tion will have been collected with which a profile of the 

 earth's surface can be drawn along a line extending from 

 Portugal through Spain, France, Belgium, Holland, 

 Germany, and Russia to Siberia, which profile would ha\ 

 the advantage that it would be unaffected by local vari. 

 tions of gravity. 



My suggestion is that enlarging cameras should !■ 

 attached to the observing telescopes, and that images of 

 the northern and southern edges of the sun and moon 

 should be thrown on sensitive films, which would have 

 be moved in a north and south direction (say at the ra; 

 of I mm. per second). An observer stationed outside th' 

 shadow would obtain a negative image consisting of a 

 black band with one gap at either edge ; the horizontal 

 (east and west) distance between the edges of these gaps 

 would be the width of the chord on the sun's disc which 

 is traversed by the moon's north or south edge; th 

 vertical (north and south) distance between the gaps woul 

 represent the time taken in traversing this chord. A:, 

 observer stationed inside the moon's shadow would obtain 

 a negative image, which would be a black band crossed 

 by a white " bend," the horizontal width of which would 

 represent the length of the chord on the moon's disc 

 traversed by the edge of the sun, and the vertical distance 

 would represent the time taken in traversing this chord. 



The length of the chord being known either by measur- 

 ment or by calculation from the time, the overlapping . 

 the northern or Fouthern edges of the sun and moon coul'i 

 easily be calculated. The moon's apparent semi-diamet- r 

 is about 1000", and its actual semi-diameter about loi . 

 miles. If the enlargement be such that i mm. = i'', th' 

 if one of the observers should find that the length of tl.' 

 chord is, say, 100 mm. on the film, equal to 100", or 

 100 miles, then the amount of overlap is 50' : 2000 = 

 1-25 mm., ", or miles, and he will know that he was 

 stationed one and a quarter miles, or 2 kilometres, from 

 the edge of the shadow. A more fortunate observer, who 

 may obtain a length of chord of only 10 mm., would know 

 that he was stationed at one-eightieth of a mile from the 

 edge. If, as seems likely, the photographic records can be 

 measured with an accuracy of one-fifth millimetre, then 

 the latter observer will feel sure of his position to within 

 one two-thousandth of a mile, or, say, to within 1 metre. 



As the angle at which the shadow strikes the earth will 

 nowhere be less than, say, 50°, it is evident that a small 

 difference of level of only a few metres between two 

 observers would make a perceptible difference in their 

 photographic records, and these would, therefore, afford the 

 means for accurately fixing their relative heights along a 

 line extending from Portugal to Siberia. Future eclipse?, 

 both total and annular, would enable a network of such 

 profiles to be determined not only across continents, but 

 also across oceans. C. E. Stromeyer. 



" Lancefield," West Didsbury, October 25. 



Khartoum for an Observatory. 



At a time when search is being prosecuted in northern 

 Africa (Nature, September 21, p. 393) for an observatory 

 site, it may not be out of place to direct attention to the 

 merits of the Khartoum vicinity, so far as this can be done 

 by one who is not an astronomer. Its features may be 

 enumerated as follows : — 



Position. — Latitude 15° 36' ; it is some degrees nearer 



