S58 SCIENCE PROGRESS 



To illustrate what is in their minds we may recall that Lord 

 Rayleigh was led to the discovery of Argon by paying attention 

 to minute discrepancies in the values he obtained for the density 

 of nitrogen from different sources ; and not only was the dis- 

 covery of Argon important in itself but it has led to others of 

 vast importance. So that all these may be said to have originated 

 in the study of a minute discrepancy between two measures of 

 what purported to be the same quantity. Is it possible that the 

 future may have in store for us similar weighty consequences, 

 traceable to the study of this discrepancy in the measure of the 

 sun's distance ? 



But astronomers know only too well how easily such dis- 

 crepancies may turn out to be due to some source of error that 

 has been overlooked. Their science is concerned, perhaps more 

 than any other science, with minute measurements which a 

 minute error will nullify or disturb ; and they must be continually 

 ready to see the edifices which they have spent some labour in 

 building tumble down like a house of cards owing to some tiny 

 flaw in the foundations. An instance of this occurred as a by- 

 product of the Oxford measures and will serve as an illustration. 

 In the year 1902 Sir David Gill made the suggestion that the 

 brighter stars were apparently rotating as a whole with respect 

 to the fainter stars as a whole, basing it upon many thousands 

 of observations made at two epochs about half a century apart. 

 If the whole universe was rotating together we might not be 

 able to perceive it. Many familiar tests would fail just as they 

 failed to reveal the rotation of the earth to our ancestors. Should 

 we have yet learnt this great fact if our sky had been permanently 

 cloudy so that we never saw the stars ? We might have sus- 

 pected it from the recurrence of daylight ; and we might have 

 actually inferred it if we could have surveyed the earth in some 

 way and found its equatorial bulge, which we might have rightly 

 ascribed to the effects of rotation. Similarly we might be able 

 to infer the rotation of the whole universe of stars if we can be 

 sure of its equatorial bulge of which the Milky Way is a possible 

 manifestation. That brilliant thinker Henri Poincare has made 

 a rough estimate of a superior limit to such a rotation in his book 

 Science et Methode (p. 285), finding a second of arc in 3,000 years, 

 or a complete rotation in four thousand million years, which is 

 really not very long considering that geologists would like to 

 take it all for the life of our earth itself. 



