Skptember. 1912. 



KNOWLEDGE. 



339 



object, houses, hills, valleys, ocean, atmosphere and 

 clouds moving together, the speed is not appre- 

 ciated : it is not even experienced. To try for a 

 sense of it, the ether-man must get away from this 

 terrestrial ball at least ten thousand miles, and then 

 the Earth would at the first glance appear motion- 

 less, in the matter of rotation. His success would 

 be no better than in his endeavour to watch the 

 speed of the Earth in its orbit. 



Hence it ma\' be inferred that although the 

 motions of the solar system, taken absolutely, quite 

 transcend terrestrial molar motions, yet considered 

 relativelv to the dimensions of the system the\' are 

 not what would be termed rapid — far from it. The 

 angular motion of a body round a centre of gravita- 

 tion is governed by the Keplerian laws : the nearer 

 to the centre, the more rapid the motion. But even 

 in the extreme case of a body revolving round the 

 Sun, just skimming its surface, and of a body doing 

 the same round the Earth, the angular speed is 

 slower than the limit fixed on. 



There is one class of bodies, of a semi-celestial 

 nature, of which no account has here been taken, 

 and which, perhaps, occupy an intermediate position. 



in the matter of motion, between the planets and 

 the Earth, viz., meteors. Here we have a com- 

 paratively minute object moving through the atmos- 

 phere at a planetar\- rate, or something approaching 

 it. Owing to comparative proximity, the great 

 speed of a meteor can better be appreciated than 

 the motion of bodies enormously further away. It 

 is far in e.xcess of the limit, the body covering 

 twenty degrees, thirt}- degrees, or more, of the field 

 of view in a very few seconds. But do we then really 

 grasp the meaning of fifty miles a second, which is 

 the speed of man)- meteors ? If one were really 

 verj- close to a meteor it could hardly be seen as it 

 passed by, except in the form of a flash of lightning, 

 because its fligb.t is so enormously swift. Yet in the 

 inter-planetary spaces, a swarm of meteors (if their 

 visibility is possible) would at the first glance seem 

 motionless, if viewed from a distance sufficiently 

 great to enable the whole swarm, or ring, to be 

 included in the field of view. 



It seems clear, then, that celestial motions take on 

 a different aspect when considered with due regard 

 to the dimensions of the bodies concerned, and the 

 distances which separate them. 



NOTES. 



ASTRONOMY. 



By A. C. D. Ckommelin. B.A., D.Sc ., F.K..\.S. 



SPECTRA OF FAINT STARS.— Not very much has 

 been done as yet under this heading, but Professor Hale (of 

 Mt. Wilson! has promised to give the preference to Professor 

 Kapteyn's selected areas in his researches. The general type 

 of spectrum of the fainter stars may be inferred by compar- 

 ing their magnitude on ordinary plates with that given on 

 red-sensitive plates. A series of the latter plates will be taken 

 at Johannesburg by Mr. Innes, and there is a prospect of a 

 corresponding series in the Northern Hemisphere. 



It is well known that the proper motions of the brighter 

 stars show a variation depending on spectral type, and it will 

 be of interest to extend the investigation to much fainter stars. 

 Miss Cannon, of Harvard, gives the following average proper 

 motions of stars of magnitude 5-00. — 



Galactic Latitude Galactic Latitude 



+ .^0° to ± 90" + 20" to - 20" 



Average Proper No. of .\\erage Proper No. of 

 Spectrum. Motion. Stars. Motion. Stars. 



B ... 0"-021 ... 1 ... 0"-028 ... 9 



B5 ... -025 ... 72 ... -024 ... 113 



A ... -062 ... 84 ... -049 ... 110 



A5 ... -101 ... 38 ... -067 ... 27 



F ... -162 ... 38 ... -091 ... 18 



Fd ... -234 ... 25 ... -135 ... 10 



G ... -308 ... 19 ... -306 ... 11 



G5 ... -357 ■ ... 25 ... -302 ... 15 



K ... -139 ... 117 ... -123 ... 61 



K5 ... -081 ... 12 ... -042 ... 6 



M ... -071 ... 15 ... -029 ... 5 



The table shows that stars of the B or helium type are 



very distant, while those of the G or solar type have much 

 larger proper motions, and are our nearer neighbours. 



RADIAL VELOCITIES.— "On Mount Wilson the focal 

 plane spectrograph has produced in a short time an astonishing 

 number of radial velocities of faint stars. The probable error 

 of velocity of a seventh magnitude star with good lines is 



0-9 kilometres per second. .\ solar star of si.xlh magnitude 

 requires twelve minutes exposure, seventh magnitude thirty 

 minutes, eighth magnitude seventy minutes." Professor 

 Pickering has been making experiments on deducing radial 

 velocities from plates taken with objective prisms, using the 

 Neodymium absorption line. The probable error is over 

 eight kilometres per second, but for statistical purposes this 

 is not serious and is counterbalanced by the wholesale 

 accunmlation of material. 



Owing to the small absolute velocity of the B or " Orion " 

 stars an attempt was made by Kapteyn and Frost to deduce 

 the solar motion from these stars alone. The curious result 

 was obtained that stars near the Apex gave a solar velocity 

 that differed ten kilometres per second from that given by stars 

 near the Antapex. It was at first surmised that this arose 

 from groups of " Orion " stars having systematic motions of 

 their own, but further research appears to have negatived this 

 suggestion, and the report now gives as the explanation the 

 shifting of spectral lines due to pressure. If this shifting is 

 really so great, it is obvious that many of the published results 

 of radial motions will need revision. 



BRIGHTNESS OF THE BACKGROUND OF THE 

 SKY. — Mr. Vntenia, of Groningcn, has made some important 

 investigations in this field. He finds that the skylight, even on 

 the darkest night, is not wholly due to starlight, but arises in 

 our own atmosphere, perhaps from a permanent aurora. In 

 spite of this obstacle, useful observations of the total amount 

 of starlight are being obtained. Professor Abbot has made 

 some observations on the top of Mount Whitney (fourteen 

 thousand five hundred feet high), to diminish atmospheric 

 illumination. The results are not yet to hand. 



It will be seen from these extracts what a large number of 

 collaborators are engaged in work on these selected areas. 

 While the full completion of the plan will be a work of many 

 years, prehminary results of interest are already appearing. 

 The whole scheme reflects great credit on Professor Kapteyn's 

 energy and foresight, and illustrates the value of method in 

 bringing about a rapid advance of our knowledge of the 

 structure of the Universe. 



It is incidentally mentioned that Professor Kapteyn is 



