January 27, 192 1] 



NATURE 



70s 



sample of paper to tearing. A compound pendulum 



of weight P and equivalent length 2 is swung always 



from the same initial angle, and in the course of its 



downward swing it tears a suitably cut sample of 



paper held partly by a clamp secured to the pendulum 



and partly by the fixed support carrying the axis. 



The pendulum rises on the far side to an angle ^,. 



When swung without tearing the paper it rises to an 



angle fj. The difference in the potential energy of 



the pendulum at the angles ^, and ^, is the work 



done in tearing the paper, and is equal to 



P/ (cos ^, — cos ^,). If the length of tear is d and the 



tearing resistance to be determined R, 



P/ 

 R = , (cos 4>i - cos 4,). 

 a 



In its present form the pendulum has the shape of a 

 segment of a circle pivoted at the -centre and cali- 

 brated round the circumference. It is held in its 

 initial position by a spring stop. A light pointer 

 mounted frictionally on the axis is set initially against 

 the stop, where it is held until the pendulum com- 

 mences its return swing ; then it moves with the 



pendulum, and so the extent of the swing is recorded. 

 The calibration can be such that R is read direct. 

 The apparatus can be seen at the office of Messrs. 

 R. J. Marx, 133-39 Finsbury Pavement, E.G. 2. 



.Among the announcements of Mr. John Murray is 

 "The Life of Alfred Newton," by A. F. R. VVollaston, 

 in which the many activities of this former professor 

 of zoology in the University of Cambridge will receive 

 attention. The work will also contain a chapter 

 describing Cambridge in the middle of last century, 

 by Sir Arthur E. Shipley, and personal reminiscences 

 of Prof. Newton by Dr. H. Guillemard. 



Errata.— Dr. T. J. J. See sends the following 

 corrections of values given by him in his letter on 

 the measurement of the angular diameter of Betel- 

 geux printed in last week's issue of Nature, p. 663 :— 

 Lines i6 and 24, "orbit of Venus" should be "orbit 

 of Mars." Lines 33-35 should read : " Sirius, which 

 is itself twenty-fivefold more luminous than our sun. 

 -Accordingly, Betelgeux gives about 6500 times the 

 sun's light." 



Our Astronomical Column. 



Pons-Wi.snecke's Couet. — The reappearance of 

 this periodical comet is expected shortly, and the 

 ensuing perihelion passage is likely to occur at nearly 

 the same time of the year as in 1869, when it was 

 on June 29, and in 1892, when it was June 30. On 

 the former occasion the comet was first seen on 

 April 9, and on the latter March 18. The following 

 is an ephcmeris : — 



Perihelion ion« 91 



1931 



Feb. 

 Mar. 



K.A. 

 h. m. 11. 



13 26 10 

 1336 II 



'3 4S 32 



14 2 o 



Uecl. N. 



1*9 8 

 2048 

 22 51 

 28 16 



Perihelioo June 13 

 " • Decl. N. 



R.A 



h. ni. %. 



•3 4S 16 

 1358 4 



14 II o 



"4 37 8 



17 8 



18 31 

 20 17 

 2459 



The date of perihelion passage is a little uncertain, 

 [so that for purposes of sweepmg two .series of posi- 

 Itions are given, one on the assumption that it will 

 — cur on June 21 and the other on June 13. The 

 met will probably be rodisrovcre<l in February or 

 (arch, and the meteoric shower connected with it, 

 Should it recur, may be expected on the night fol- 

 ing June 27. 



Thb Magellanic Clouds. — Dr. E. Hertzsprung 



a paper on these clouds in Monthly Notices 



ol. Ixxx., No. g). It is basrd on Mr. R. E. Wilson's 



neasures of the radial velocity of nebulae (Lick Obs. 



"libs., vol. xiii.). On the whole, Mr. Wilson found 



velocities for the gaseous ncbuls. There were, 



>rever, notable exceptions in the cases of seventeen 



ebuls in the greater cloud, which showed average 



recessions of 276 km. /sec, and one in the lesser cloud 



which showed 168 km. /sec. It was a reasonable 



inference that the clouds were external to our galaxy ; 



indeed, thi.s was already thought probable on other 



-ounds. On studying the individual motions it was 



n that they showed a steady upward tendency as 



■ went southward. Mr. Wilson suggested in fx- 



■ nafion a rotation of the cloud; Dr. Hertzsprung 



tits out that it is simpler to assume that the varla> 



•1 is a perspective effect, due to diffrrent parts of 



...•■ rloud being nt different aniTuIar distances from 



the .-iprx of motion. He applies analvsis to thie 



ob<ifrv;itions, .ind finds as the most probable solution 



NO. 2674, VOL. 106] 



that the velocity of the greater cloud (corrected for the 



solar motion) is 608 km./sec. towards R.A. 4h. 31m., 



S. decl. 4° 42' (galactic long. 168°, S. lat. 30°). The 



single nebula observed in the lesser cloud gives a 



result consistent with the two clouds having the same 



motion. The greater cloud is 637° from the apex, 



which gives a velocity of 560 km./sec. at right angles 



I to the line of sight. Taking the distance as 10,000 



. parsecs, this implies an angular velocity of somewhat 



j more than a second per century, the position angle 



I being 348° and 55° for the greater and lesser clouds 



i respectively. 



It will be remembered that Dr. Hertzsprung was 

 one of the first to give a trustworthy estimate of the 

 distance of the clouds, based on methods quite similar 

 to those applied later to the globular clusters by Dr. 

 Shapley. 



Investigation of the Einstein Spectral Shift. 



Bulletin No. 64 of Kodaikanal Observatory contains 

 a full account of a second investigation of this effect 

 by Mr. J. Evershed. Mr. Evershed's former result 

 (Bulletin No. 39) indicated considerably more than 

 half the shift predicted by Einstein. In view of Dr. 

 St. John's much smaller result, he planned a more 

 extensive research, using the .Anderson 6-ln. grating, 

 which was carried out in the spring of 19 18. The 

 spectrum of the sun both near the centre and near 

 each pole was compared with the carbon arc. The 

 results are corrected for solar and terrestrial rotation 

 and the earth's radial velocity. The lines used are 

 not the same as those of Dr. St. John, but the 

 strongly marked triple carbon bands extending from 

 3876-59 to 3881 78. The mean values of the shift, 

 sun minus arc, are : 



Sun's centre ... -^00037 A. 



(omitting one discordant day) 

 .Sun's north limb ... -ho-oo7i A. 

 .Sun's south limb ... -fo-oioo A. 

 Einstein's value ... 4-00082 A. 



Mr. Evershed notes that the late Mr. Pocock found a 

 similar discordance between the measures at the sun's 

 north and south limbs. 



