188 



KNOWLEDGE 



[August 2, 1897. 



the influence of extern bodies is comparatively small. In 

 consequence of the law of gravitation, all stars appear to 

 form a physical system in one sense ; but within this vast 

 system there are smaller ones like the solar system, or 

 those revealed by clusters and binary stars. But to con- 

 stitute a system in this latter sense the stars must be mainly 

 influenced by each other, and therefore they must be — like 

 the members of our solar system — nearer to each other 

 than to any other stars, or else their masses must be such 

 that notwithstanding their distances from each other they 

 are the main causes of each other's motions. Now I think 

 when we find stars whose parallaxes, though not accurately 

 known, are certainly very small (and I may add cannot be 

 shown to be approximately equal), situated at considerable 

 angular distances from each other on the sphere, the 

 probability is strongly against their forming a system in 

 this sense. The chances are that, under the law of gravity, 

 some of them are influenced more largely by stars not 

 included in the supposed system than by those that are so. 

 But perhaps Mr. Proctor and Mr. Backhouse do not mean 

 that the stars in Ursa Major form a system in the same sense 

 as the solar system, but rather that they had a common 

 origin. That stars with a common origin may afterwards 

 become detached from each other, and wander as practically 

 independent units through space, seems very probable, 

 and their proper motions may indicate the region of the 

 sky in which they were once foimd together. But suppose 

 the paths of these five stars which Mr. Backhouse has 

 given in your last issue, were meteor-tracks observed on a 

 night when meteors were as numerous and as variable in 

 direction as the stars whose proper motions are known — 

 could he confidently refer the five meteors to the same 

 radiant, and define its position ? I agree with Mr. Back- 

 house as to the importance of the direction of the motion. 

 Except in the case of a principal star and satellite, I think 

 in order to prove the existence of a physical system the 

 agreement in direction should be very close. But I do not 

 think an agreement in small motions affords the same 

 evidence of physical connection as an agreement in large 

 ones. We might infer with probability a relationship 

 between two men of six feet six who bore a general 

 resemblance to each other, but the probability would be 

 much less in the case of two men of five feet six. There 

 are perhaps one thousand of the latter to one of the 

 former, and when the numbers are great, chance coin- 

 cidences must occur. Mr. Backhouse's simile of the 

 meteor swarms is hardly apposite. Meteors are so small 

 that they have very little effect on each other's motions — 

 unless they should get into a part of space where there 

 were no large bodies. They keep together because the 

 initial motion and the attracting forces are nearly the 

 same. Two swarms might cross each other almost without 

 disturbance, because the mass of each is very small. But 

 I do not see how two star systems could cross each other 

 without profound disturbance. 



Now to turn to Mr. Hardy. I quite concede that Dr. 

 Croll would have proved his case if he had shown that a 

 maximum eccentricity combined with a winter aphelion 

 would produce a permanent icecap at the North Pole — 

 meaning thereby an icecap of a more permanent character 

 and of greater extent than if the eccentricity of the earth's 

 orbit were at all times inappreciable. This is what, in my 

 opinion, he has failed to prove, as completely as the late 

 Mr. .J. J. Murphy failed to prove that the effect in question 

 would be produced by a maximum eccentricity combined 

 with a winter jierihelion. 



I almost think Mr. Hardy has inserted the word " not " 

 by inadvertence in the paragraph ; " Mr. Monck's argument 

 that the greater heat of a short summer sun would have 



the effect of melting the snow and ice is not borne out by 

 practical observation, as although the Polar sun shines on 

 the Polar snow for six months it has very little effect on 

 it." Surely Mr. Hardy does not regard the Polar summer 

 as an example of the expenditure of a large quantity of 

 heat in a short time, which is the thing that I was dealing 

 with. As to the melting of the Alpine snows, I would 

 ascribe it to the sun, though the water, on being formed 

 from the snow or ice, naturally sinks and makes its 

 appearance below the glacier. I do not believe in any 

 such permanent earth-heat (unless in the case of volcanoes) 

 as would constantly melt the lower stratum of snow or ice 

 without any aid from the sun. But in any event we have 

 not in this case an example of the effect of shortening the 

 time of expending the heat without any diminution in the 

 quantity. I would be disposed to think that the shorter 

 the time the greater would be the melting efi'ect. This 

 might be tested on a small scale by experiment, but I am 

 not aware that the attempt has been made. Divide a 

 block of ice into two equal parts. Expose one to a given 

 number of heat units in an hour and the other to the same 

 number of units in five minutes. In which case will the 

 largest amount of water be produced from the ice ? Dr. 

 Croll and Sir Eobert Ball might, I think, have tried this 

 instead of arguing it. Use, for example, two lamps, one of 

 which hag been found to give twelve times as much heat 

 as the other (though if the chamber was not kept steadily 

 at freezing-point the experiment would not be quite a fair 

 one). 



Geologists are not astronomers, but they may discover 

 facts which efl'ectually dispose of astronomical explanations 

 of the phenomena of their science. W. H. S. Monck. 



\_No further corres^omJence on this subject can be pnbtlnhed. — Ec] 



A NATIONAL ASTRONOMICAL MUSEUM. 

 To the Editors of Knowledge. 



Sirs, — Permit me through your columns to call the 

 attention of those interested in scientific education to the 

 desirability of a National Astronomical Museum, with 

 working models, for the purpose of popularizing that 

 science of which the British public are the most ignorant 

 of publics, and of which the merest sketch is given in ele- 

 mentary schools. 



English Governments have always had a great objection 

 to undertake the installation of museums for the diffusion 

 of knowledge, or to advance funds to that end; «,//., the 

 foundation of the Hunterian Museum. But the expense 

 of founding such a museum as I suggest would not be 

 great ; a small dynamo would work a large number of 

 models, and I am certain that many persons interested in 

 astronomy would subscribe to provide a fund to support 

 such an institution. I have received letters from several 

 of the most distinguished astronomers of England, among 

 others from Sir Robert Ball, promising hearty support. 



What we have in this connection in London is valueless 

 to the masses, and nearly unattainable. For example. 

 South Kensington, and also Mr. Ledger's Gresham Lectures 

 crowded within four or five days once a year. 



I conceive a free museum open during the day and the 

 evening, Sundays included, where a curator would explain 

 the working models. There should also be an amphi- 

 theatre, where — I have reason to know — the most illus- 

 trious astronomers would willingly give occasional lectures, 

 illustrated by practical slides. An entrance fee to these 

 lectures is an after question, but the museum must be free — 

 except, perhaps, on students' days. The institution should 

 not be farther from the dwellings of the working classes 

 than the British Museum. 



Some years ago I commenced putting my idea into 



