266 



KNOWLEDGE & SCIENTIFIC NEWS. 



[Nov., 1904. 



ASTRONOMICAL. 



The Nebulae and the Milky Way. 



It is oRen stated that the iiel)ul;e piopniy bn called — the 

 "white" nebulie — avoid the galactic region and cluster 

 towards its two poles. It is manifestly so with respect to the 

 north pole of the Galaxy, but Dr. C. Easton, in the Astroiio- 

 iiiisclu' Sdihyiihtcn (No. J969I. questions if the same relation 

 holds good for the southern pole. The statistics at our 

 disposal do not bear it out, but this has hitherto been generally 

 explained as due to the insufficiency of obser\ations in the 

 southern hemisphere. Dr. Easton shows that this is not the 

 case ; for since the Galaxy is inclined at an angle of 6o- to 

 the equator a considerable part of the galactic northern 

 hemisphere lies to the south ot the equator, and of its southern 

 hemisphere to the north. Calling the former segment A, the 

 latter B, we ha\e 



Faint nebuhe 

 Bright nebuUe 



B 



A 754 a 1043 

 A 152 B 71 



If nebulas were really distributed on the whole in the same 

 way in the two galactic hemispheres, and if an}- apparent want 

 of the expected condensation round the south galactic pole 

 were simply due to the insufficiency of observations in the 

 southern hemisphere, then more nebula;, both bright and 

 faint, should be observed in segment B than in segment A. 

 There is an increase in faint nebuUe, but a most striking 

 falling off in l>riglit. This indicates a great increase in the 

 proportion of faint to bright nebuke in the galactic southern 

 hemisphere, and it is exceedingly improbable that future 

 observations in the southern hemisphere will discover so large 

 a number of nebuUe round the south galactic pole as to bring 

 it into symmetry with the northern. The distribution in the 

 tw o hemispheres appears to be different, and Cleveland .Abbe's 

 theory of an •' ellipsoid of nebulas" with its major axis at right 

 angles to the galactic plane, seems to accord with this want of 

 symmetry. Dr. Easton, in the concluding portions of his 

 paper, regards the faint nebulosities as allied to the stellar 

 agglomerations of the Milky Way ; the nebuke properly so- 

 called to the sparsely distributed stars of the general stellar 

 system, the non-galactic stars. 



* * * 



Explanation of the Martian a^nd L\ina.r 

 Canals. 



Professor W. H. Pickering, in Popuhir A^trvnninw dccWnas 

 to follow- Mr. P. Lowell in his heroic scheme of artificial 

 pumping to account for the flow of water in the Martian 

 canals. He shows that the lunar canals are dotted by small 

 craterlets, and are so symmetrically connected with them as 

 to show a causal connection. Pie therefore suggests that the 

 canals on the Moon, and by analogy on Mars also, are lines of 

 volcanic action where the crust has been fractured, and that 

 enough water and carbonic acid may escape from the centre 

 craterlet and fiow dow-n its sides to de\'elop the vegetation 

 upon its slopes, whilst the smaller quantities escaping from 

 various points along the radiating cracks similarly de\-elop the 

 vegetation along their course, the " lakes " and " canals " as we 

 see them being thus regions of vegetation. On account of the 

 rarity of the atmosphere, the vapours, instead of rising, would 

 immediately spread themselves along the surface of the ground. 



The Monthly Rcvici^ for October contains an article on 

 "The Markings on Mars: a Plea for Moderate Views," by 

 Major P. B. Molesworth, R.E. Pew astronomers can speak 

 on the subject of Mars with the authority of .Major Molesworth, 

 since his studies of the planet ha\e been carried on under 

 exceptionally good observing conditions, with the utmost per- 

 severance, and with great skill in delineation. He concludes 



that : (i) The markings on the surface of the planet are more 

 or less permanent, ^but subject to minor changes. (2) Their 

 intensity depends in some way on the Martian seasons. 

 (j) The structureof the delicate detail is the same all over 

 tne planet, both in the light and dark areas, the only difference 

 being in the varying tone of the " background." (4) This 

 detail is " the integration of markings far too small to be 

 separately defined." 



* * * 



The Ninth Satellite of Saturrv. 



'I he first visual observation of this object was obtained on 

 August S by Professor 1*^. K. Barnard at 18 hrs. o min. G.M.T. 

 Itsappareut place was K.A. 21 hrs. 23 min. i-osecs. ; declination, 

 lO ' 36' S". On September 3 Professor Barnard found no star 

 was visible in this place. The magnitude of the object was 

 estimated at i5'5 or i6-o. Another observation by Professor 

 Barnard is dated September 12, when the magnitude of the 

 satelhte was given as 16-7. 



In the number of the Ubscrvatoiy for October, Mr. Cromiuelin 

 gives the result of a rough preliminary examination of the 

 orbit from the very few observations which have yet been 

 published. He finds that the hypothesis of retrograde motion 

 suits the observations as given much better than that of direct 

 motion. This, if established, would be a most extraordinary 

 circumstance, the other eight satellites moving directly. Mr. 

 Crommelin gives the siderial period as 443 days, and the 

 distance 6,gOo,ooo miles, inclination to ecliptic about 6°, to 

 Satyrn's orbit about 4j '. to Saturn's equator, 30 ". The magni- 

 tude given by Professor Barnard, i6-7, would correspond to a 

 diameter of about 120 miles. 



* * * 



Radiation in the Solar System. 



In the course of an address given to the British .Association, 

 Professor J. H. Poynting gave in clear and succinct form some 

 of the conclusions which may be drawn from researches in 

 recent years, both on the temperature effects of radiation and 

 the effects due to light pressure. Beginning with Stefan's 

 law, that the stream of energy is proportional to the fourth 

 power of the temperature, reckoned from the absolute ^ero 

 273 ' below freezing point on the Centigrade scale, he quoted, 

 as probably not far from the true value, that the stream of 

 radiation from the sun falling perpendicularly on i sq. cm. 

 outiidi: the earth's atmosphere would heat i gramme of water 

 5*f" C. every second, or would give ^f caloric per second. 

 Hence he deduced that the mean temperature of the sun's 

 radiating surface is 6000 ', if the sun radiates as a body would 

 do which is perfectly black when cool. Further, he gave a 

 table of temperatures at various planetary distances from the 

 sun's centre : — 



Distance from the Temperature 



Sun's centre. Centigrade. 



At Mercury's distance. . . . 210° Tin nearly melts. 



At V'enus's distance 85° .Alcohol boils. 



At Earth's distance 27^ Warm summer day. 



At Mars distance — 30 Arctic cold. 



At Neptune's distance .. —219' Nitrogen frozen. 



Now, the estimated mean temperature of the earth's surface 

 is about 16° C, which is sufficiently ne.ar the value given in the 

 table when the radiation from the surface of the atmosphere 

 and other such conditions are taken into account. Professor 

 Poynting then points the moral that, given atmospheric con- 

 ditions on Mars not very unlike those on the earth (as obser- 

 vation seems to show), even the highest equatorial 

 Martian temperature cannot be much greater than —38", and 

 •' it is hard to believe that he can have polar caps of frozen 

 water melting to liquid in his summer and filling rivers or 

 canals. Unless he is very different from the earth, his w-hole 

 surface is below the freezing point." 



Turning, then, to the effects of light pressure, he gives the 

 total eftect on the earth at its present distance from sunlight 

 as 70,000 tons. Since gravitation depends only on the mass 

 and light pressure on the surface area, it follows that were 

 the earth's volume divided up into separate spheres, each 

 irrJoo cm. in diameter, the pressure of light would balance 

 the pull of gravitation. If, on the other hand, we diminish the 

 radiating body whilst retaining its high temperature, we find 

 similar effects. If it were possible to reduce the sun to a dia- 



