438 SCIENCE PROGRESS 



control ? Scientific workers are no longer few and isolated ; 

 they are numerous and they are binding themselves into 

 organisations among which that for making the Great Star 

 Map has an early and an honourable place. It does not seem 

 unreasonable that the changed conditions should leave their 

 mark on the methods of work, and that the relation of cost to 

 value of product should be considered as in other enterprises. 

 In old days the value of the product was so high that any 

 cost could be neglected ; and there are still cases where this 

 is the correct view — let us hope there always will be. But in 

 the case of a great piece of straightforward measurement like 

 the Star Map, the value can be expressed very definitely 

 by the "probable error" of the result; and alternative plans 

 for the work can be compared by setting down the probable 

 error and the cost (in time and money) of each. So far as I 

 know, however, this principle has not yet been applied except 

 in a special case in geodesy. It is certainly one that may be 

 applied in other sciences if judged sound ; but at present it 

 scarcely seems to have met with the approval, even the atten- 

 tion, of any large body of scientific workers. I trust no apology 

 is needed for inviting attention to it in a scientific review of 

 this kind, even at the expense of a slight digression. 



Let us now consider what is to be the outcome of this 

 immense piece of work. What are we likely to learn from 

 these millions of measurements ? As already stated, the 

 interest will come when it is repeated — in the study of the 

 movements of the stars ; which are so minute that, as a rule, at 

 least a century is required to discern them even by our improved 

 modern methods. The movements are not really slow ; we 

 may take the velocity of our earth in its annual journey round 

 the sun — about 20 miles a second — as a fair sample of the 

 velocities of the stars. But our great journey from side to side 

 of the sun (nearly 200,000,000 miles across) would seem a 

 minute movement to the nearest star, and to the great majority 

 would be imperceptible. This is however not the only 

 movement of the earth ; the sun himself is moving and we 

 partake of that motion also. It is not a circling or oscillatory 

 motion, but is in the same direction year after year, so far 

 as we can at present ascertain, the distance traversed each 

 year being about 400,000,000 miles. One year's journey is 

 therefore scarcely more perceptible from the distant stars than 



