970 



SCIENCE. 



[N. S. Vol. XV. No. 390. 



recorded in our sacred books." Thus 

 easy is it to mistake the order of a quan- 

 tity for the quantity itself. 



When we pass from terrestrial limita- 

 tions to celestial phenomena the field for 

 measurement and calculation is immensely 

 enlarged, though the results attainable are 

 less easy of ready appreciation. The 

 Jovian, the Saturnian and the Martian 

 subsystems, which have been pretty thor- 

 oughly explored by the observer and the 

 computer, present to us the type, appar- 

 ently, not only of the solar system, but of 

 the galaxy of systems within telescopic 

 view. And the surveys of the heavens now 

 in progress indicate likewise that isolated 

 stars are the exception rather than the 

 rule, and that the visible stars are gener- 

 ally attended by one or more satellites, 

 which are probably oftener dark than 

 bright bodies. Visual and photographic 

 measurements have, in fact, united in re- 

 cent years in the demonstration that the 

 number of material bodies in the universe 

 is enormously greater than we have hith- 

 erto imagined. Here again, however, as 

 in the case of the geological phenomena 

 just referred to, we must be content to a 

 great extent for the present with a knowl- 

 edge of the order of the quantities meas- 

 ured and calculated. But to be able to 

 state what is the order of the distances 

 which separate the fixed stars from one 

 another, the order of the volume of the 

 visible universe, the order of the quantity 

 of mass in that volume, and the order of 

 the time unit requisite for the expression 

 of the historical succession of celestial 

 events, seems little short of a stupen- 

 dous contribution to knowledge when one 

 reflects on the obstacles, material and intel- 

 lectual, that have stood in the way of its 

 attainment. 



The distances asunder of the stars are so 

 great that the hundred and ninety odd 

 millions of miles in the diameter of the 



earth's orbit about the sun make an in- 

 conveniently small base line for the meas- 

 urement of the least of those distances and 

 a hopelessly inadequate one for the meas- 

 urement of the greatest of them. It would 

 appear more fitting, in fact, to express such 

 distances indirectly in the number of years 

 it takes light moving at the rate of 300,- 

 000 kilometers per second to traverse them. 

 Assuming with Lord Kelvin that the visi- 

 ble universe is comprised within a sphere 

 whose radius is equal to the distance of a 

 star whose parallax is one thousandth of a 

 second, this distance would require light 

 about three thousand years to pass over it, 

 while the average distance asunder of the 

 visible stars is considerably less, but still 

 of the same order. Lord Kelvin has shown 

 also in a profound mathematico-physical 

 investigation recently published* how we 

 may assigii limits to the amount of mass 

 in the visible universe. It appears from 

 this investigation that there are something 

 like a thousand million masses of the mag- 

 nitude of our sun within that universe. 

 The figures for this amount of mass have 

 little meaning to most of us when expressed 

 in ordinary units. The mass of the earth, 

 for example, with its 6,000 X 10" metric 

 tons,f is a mere trifle, for the sun has 

 about 327,000 times as much mass as the 

 earth. The mass of the sun therefore is 

 the obviously convenient unit in this case; 

 and we have only to imagine our solar 

 system surrounded by a thousand million 

 such suns, each in all probability attended 

 by a group of planets, to get a sufficiently 

 clear idea of the quantity of mass within 

 visual range of our relatively insignificant 



*'0n Ether and Gravitational Matter through 

 Infinite Space,' Philosophical Magazine, August, 

 1901. ' On the Clustering of Gravitational Mat- 

 ter in any Part of the Universe,' Nature, Vol. 

 64, No. 1669. 



t The metric ton of 1,000 kilograms, or 2,205 

 pounds, is about the same as our ' long ton ' of 

 2,240 pounds. 



