5o6 



NATURE 



[June 23, 1910 



those actually observed, we also receive radiations, such 

 as the Rontgen rays, for example, which are not deflected 

 by the earth's magnetic field, as they do not carry electric 

 diarges. Their chief elTect will be to ionise the gases 

 of which the atmosphere is composed, i.e. make the air a 

 better conductor of electricity. Ultra-violet light has the 

 same effect. It is known that a small part of the mag- 

 netic forces acting on a compass needle is due, not to 

 the magnetism or electric currents below the earth's 

 surface, but to electric currents already existing in the 

 atmosphere, and which the speaker showed were brought 

 about by the atmosphere cutting across the earth's lines 

 of magnetic force in its general circulation around the 

 globe. If the regions of these upper electric currents are 

 at any time made by some cause more conducting, elec- 

 tricity will be immediately set in motion, which in turn 

 affects our compass needles. This new theory, called " the 

 ionic _ theory of magnetic disturbances," satisfactorily 

 explains the principal features of magnetic storms. As 

 the currents get lower down in the atmosphere their 

 velocity is checked, so that, instead of taking but three 

 to four minutes to circulate around the earth, as do the 

 higher currents, it may take them half an hour and more ; 

 however, their actual effect on the magnetic needle would 

 be greater, because of their coming nearer to the earth. 

 The theory also opens up the possibility of accounting for 

 some of the other changes and variations experienced by 

 the earth's magnetism, and likewise has a bearing on the 

 peculiar formation of the magnetic fields in sun-spots dis- 

 covered by Prof. Hale. 



Magnetic results of the first cruise of the Carnegie, Dr. 

 L. A. Bauer. The non-magnetic vessel Carnegie com- 

 pleted on February 17 the first cruise, covering in all, 

 since September i, 1909, 8000 miles. Special tests made 

 at Gardiner's Bay, Long Island, and at Falmouth, England, 

 proved conclusively that there are no corrections to the 

 magnetic instruments of the kind encountered on vessels 

 in which more or less iron occurs in the construction. 

 Thus in a single voyage errors could be disclosed in the 

 compass charts used by mariners on their Transatlantic 

 voyages between New York and England of importance, 

 not alone from a purely scientific standpoint, but from 

 that of practical and safe navigation as well. The errors 

 found by the Carnegie in the declination at various points 

 along the track followed by the vessel amounted, on the 

 average, to about i degree— an error which persisted in 

 the same direction for long distances. After leaving 

 Falmouth the Carnegie headed for Funchal, Madeira. 

 From thence she sailed to Bermuda, and finally arrived 

 at Brooklyn on February 17. In spite of the unusually 

 adverse conditions frequently met with during this first 

 cruise, more or less extensive magnetic observations were 

 secured almost daily. The errors of the compass charts 

 were found, in general, even mo.-e pronounced for the 

 southerly half of the cruise, viz. Madeira to Bermuda, 

 than for_ the northerly half, and were again shown to be 

 systematic in their nature. Some of the charts were in 

 error 2 to 3 degrees. For the entire cruise important 

 corrections were also disclosed for the magnetic charts 

 which give the lines of equal magnetic dip and of equal 

 magnetic force. The Carnegie is now being fitted out for 

 a circumnavigation cruise of about three vears. In the 

 nreantime, the magnetic surveys of unexplored countries 

 are pushed on, so that it is confidently expected that bv the 

 year 1915 the general magnetic survey of the greater' part 

 of the globe will have been completed in sufficient detail 

 to permit the construction and issuing of a new set of 

 magnetic charts. 



On the distances of red stars. Prof. H. N. Russell. 

 Comparison of the parallaxes of stars, derived by the 

 author from photographs taken at the Cambridge Observa- 

 tory (England) by Mr. A. R. Hinks and himself, and their 

 spectra, determined at Harvard under the direction of 

 Prof. Pickering, shows a marked correlation between 

 spectral type and parallax. The proportion of orange and 

 red stars (types K and M) among those of large proper 

 motion, and especially among those shown bv direct 

 measurement to be our near neighbours, is very much 

 greater than among the general run of stars of the same 

 apparent brightness. Conversely, stars of the same 

 apparent brightness and proper motion are, on the average, 

 nearer to us the redder they are. It follows that these stars 

 NO. 2 12 I, VOL. 8.^1 



are intrinsically fainter the redder they are, the reddest ones 

 being, on the average, only one-fiftieth as bright as the sun. 

 On the other hand, many bright red stars (such as .\rclurus) 

 are at great distances, and are actually at least 100 times as 

 bright as the sun. All this can be explained on the hypo- 

 thesis (now well established on other grounds) that the 

 reddest stars are the lowest in temperature. With the 

 latest determinations of temperature and surface bright- 

 ness, it appears that the fainter red stars are somewhat 

 smaller, and presumably denser, than the sun, while the 

 brighter ones are very much larger than the sun, and 

 presumably of very small density. The latter class prob- 

 ably represent an learly stage of evolution, and the former 

 the latest stage that can be observed. 



A standard system of photographic stellar magnitudes. 

 Prof. E. C. Pickering. Since 1879 about two million 

 photometric observations of one hundred thousand stars 

 have been made at the Harvard College Observatorv. 

 The results, published in vols. 1., liv., and Ixx. of the 

 " Harvard Annals," furnish a standard scale for deter- 

 mining the brightness of the stars in all parts of the sky, 

 according to a uniform system. The general introduction 

 of photography in nearly all departments of astronomy 

 has created an urgent need for a similar scale to give the 

 photographic magnitudes of the stars. The two scales 

 will differ, since red or yellow stars will always give fainter 

 images. The scale proposed will be the same for white 

 stars as the visual scale. Three methods are adopted in this 

 work for determining the photographic brightness — fir^ 

 correcting the visual magnitude by the class of spectrur. 

 secondly, by measuring with great care the photographic 

 brightness of a sequence of stars near the North Pole, and 

 superposing this photographically on the stars to be 

 measured ; thirdly, by attaching to the object-glass of the 

 telescope a small prism, a second image of each star, five 

 magnitudes fainter than the principal image, is formed. 

 All three of these methods are in use on a large scale at 

 the Harvard Observatory, and it is hoped that, as the 

 result of many thousand measures, a satisfactory solution 

 of the problem will be found. 



Some interesting double stars, Prof. Eric Doolittle. The 

 many thousand double stars in the sky may be divided 

 into two classes. There are some in which the two stars; 

 are not really near each other, but merely happen to liei 

 in the same direction as viewed from the earth, and there' 

 are others which form true systems composed of two suns' 

 revolving about their common centre of gravity. In the! 

 latter case, measures show that one sun revolves about the' 

 other in an elliptic orbit. It often happens that a very 

 few measures of such a system secured at certain critical 

 times throw unusual light on the nature of the motion and 

 the size of the orbit. This is especially the case when the] 

 companion star apparently ceases its motion in one direc-j 

 tion and begins to move backward, and also when the! 

 companion is passing nearest the principal star. Several, 

 diagrams were shown describing measures of this kind 

 wiiich had recently been secured. An account was alsoj 

 given of the discovery of a very close double star during! 

 its occupation bv the moon. j 



THE ROYAL OBSERVATORY^ GREENWICH.' 



"O EAD at the annual visitation of the Board of Visito; 

 ■^^ on Saturday last, the Astronomer Royal's report < 

 the work done at the Greenwich Observatory during tl. 

 year ended May lo contains many items of general interet" 



On the transit circle the new central illumination w.i 

 further compared with the annular illumination of the fiel' 

 and it was found that, with the latter, transits were observeu 

 0-24S. earlier than they are' with the new form. Six stars! 

 from Newcomb's Fundamental Catalogue are observed each 

 night in order to connect the observations of the reference 

 stars for the Oxford Astrographic Zones with any system 

 thyt may ultimately be adopted by the Permanent Inter- 

 national Committee. 



Observations of the diurnal changes of level and nadir 

 during 1909 showed that changes of level were less, and 

 of nadir slightl)' greater, than the means for 1897-190^. 

 The lunar observations with transit circle and altazimuth 

 are in good agreement, and, for 1908, show the nienni 

 error of the tabular place to be — o-395s. in R.A. andi 



