August 25, 192 1] 



NATURE 



825 



thus behaves as a solid. The addition of an artificial 

 head to the top surface of the same clay increases 

 the pressure of fluidity bv about 7 per cent, when 

 the addition is 200 per cent, of the actual head. 

 The experiments on discharging clay under pressure 

 through sharp-edged circular orifices are also of in- 

 terest. The rate of discharge increases more rapidly 

 than the rate of increase of pressure, and ultimately 

 there is a phenomenon analogous to the pressure of 

 fluidity. Reducing the size of the orifice, keeping 

 the pressure constant, reduces the discharge per unit 



area of orifice. The initial pressure necessary to 

 cause the discharge to begin increases considerably as 

 the diameter of the orifice is decreased. Practically 

 the same result is obtained whether a disc or a sphere 

 is used in determining the pressure of fluidity, and 

 the result is independent of the diameter of the disc 

 or sphere within a considerable range. Mr. Acker- 

 mann's work on this subject shows promise of great 

 value to engineers in dealing with foundations and 

 retaining walls, and we trust that his experimental 

 work will be continued. 



Our Astronomical Column. 



The Recent Meteoric Display. — Mr. W. F. 

 Denning writes that further proof of the unusually 

 abundant display of August meteors is provided by 

 Mr. S. B. Mattey, observing at St. Helier, Jersey, 

 on August II during the quarter of an hour between 

 i4h. and i4h. 15m. G.M.T., who saw sixty-two 

 meteors. This indicates a rate of about 250 per hour, 

 and proves that the shower was witnessed in extra- 

 ordinary activity. About 25 per cent, of the meteors 

 seen by Mr. Mattey were bright ones, exceeding stars 

 of the first magnitude. Their light was frequently 

 strong enough to illumine buildings near his place of 

 observation ; in fact, he describes the effect as being 

 somewhat similar to that occasioned by so-called 

 sheet lightning. 



Detection of Encke's Comet. — A letter from Mr. 

 J. F. Skjellerup, dated Capetown, July 29, announces 

 that he and Mr. W. Reid detected Encke's comet on 

 July 27 at 5h. 15m. G.M.T., when it preceded 

 19 Sextantis by 31 seconds, and was 2' to the south 

 of it, which makes its apparent position R.A. 

 loh. 8m. IIS., N. decl. 4° 58'. The estimates of its 

 magnitude by the two observers were 95 and brighter 

 than 8-0. 



The following elements were predicted by Mr. 

 Matkiewitch : — 



T = 1921 July 13-28 G.M.T. 

 «= 184= 43-5' 



^ = 334°35"5' 

 i = 12° 3ri' 

 log a =0-34598 

 e =0-84671 

 log q =9"53I49 

 The above observation would indicate a value of T 

 some 02 day earlier than the prediction. 

 The comet will be 192 1 d. 



The letter states that Pons-Winnecke's comet was 

 observed at midnight on July 27, in R.A. ih. 24m., 

 S. decl. 38°, magnitude about 85. 



Study of the Moon's Surface. — Mr. Walter Good- 

 acre has just brought out the eighth report of the 

 Lunar Section of the British Astronomical Associa- 

 tion. He dwells on the immense value in seleno- 

 graphy of the splendid photographs taken by Mr. 

 F. G. Pease with the loo-in. Mount Wilson reflector. 

 He states that they show more detail than a 6-in. 

 visual telescope would do, even with the best seeing. 

 The report contains several charts showing on a 

 larger scale much of the detail that has been detected 

 on the photographs. One is of the " Straight Wall 

 near Thebit, " showing that it is really by no means 

 straight. Enlargements of the craters Ptolemaus, 

 Clavius, Copernicus, Arzachel, Gassendi. etc., show 

 much new detail, mostly of the nature of tiny craters 



NO. 2704, VOL. 107] 



or narrow clefts. Mr. Goodacre considers that the 

 new evidence is unfavourable to the theory of meteoric 

 formation of the lunar features. Various fine details 

 are noted, in particular an apparent landslip on the 

 wall of Birt A. 



Mr. J. W. Durrad contributes a fine drawing of 

 Gassendi, showing numerous clefts on the floor, some 

 of which are new. 



The Distances of the Globular Clusters. — The 

 Bulletin of the National Research Council, Washing- 

 ton, D.C., for May last contains an interesting dis- 

 cussion between Dr. Harlow Shaplev and Prof. H. D. 

 Curtis on this subject. Taking the Hercules cluster 

 as an example, they contend respectivelv for 36,000 

 and 3600 light-years as its distance. The strongest 

 argument for the former distance is the presence of 

 B stars in the cluster and the demonstration that the 

 average absolute magnitude of such stars is zero or 

 brighter, judging from the stars in proximity to the 

 sun. Prof. Curtis prefers to work from the average 

 absolute magnitude of all stars within measurable 

 distance, but/ Dr. Shapley replies that the average is 

 itself a function of distance, since the stars that are 

 really very faint are altogether lost to view at moderate 

 distances. 



Another point discussed is the correlation between 

 period and absolute magnitude in the Cepheid 

 variables. Prof. Curtis gives a diagram shotting that 

 the case for this correlation becomes much less con- 

 vincing than Dr. Shapley had supposed, when the 

 number of galactic Cepheids employed is increased. 

 Dr. Shapley replies that he used the Cepheid method 

 solely as corroborative of several others, and that the 

 strongest argument for the correlation is in reality 

 deduced from the fact that the methods all fall into 

 line so well. 



The discussion also involves the status of the spiral 

 nebulae. Dr. Shapley 's estimate of the size of our 

 Galaxy is so great that if the spirals were similar 

 objects they would be so remote that we could not 

 expect to see novae in them. From the fact that 

 several novae have been detected he concludes that 

 they are nof stellar, but actually formed of diffused 

 matter. Prof. Curtis 's smaller galactic diameter 

 permits the view that the spirals are external galaxies. 

 He estimates the distance of the Andromeda nebula 

 (supposed to be the nearest spiral) as 500,000 light- 

 years, and invokes the presence of a zone of occulting 

 matter near the galactic plane to explain the observed 

 distribution of the spirals. 



The discussion is highly instructive, and the method 

 of putting the two views of such difficult questions 

 side by side is most helpful as a check on over-hasty 

 deductions and a test of the weaker links in a chain 

 of evidence. 



