410 THE PRINCIPLES OF SCIENCE. [chap. 



In a second class of cases, the event A may make our 

 perception of B follow, which would otherwise happeti 

 without being 'perceived. Thus it was believed to be the 

 result of investigation that more comets appeared in hot 

 than cold summers. No account was taken of the fact 

 that hot summers would be comparatively cloudless, and 

 afford better opportunities for the discovery of comets. 

 Here the disturbing condition is of a purely external 

 character. Certain ancient philosophers held that the 

 moon's rays were cold-producing, mistaking the cold 

 caused by radiation into space for an effect of the moon, 

 which is more likely to be visible at a time when the 

 absence of clouds permits radiation to proceed. 



In a third class of cases, our perception of A may make 

 our perception of B follow. The event B may be con- 

 stantly happening, but our attention may not be drawn to 

 it except by our observing A. This case seems to be 

 illustrated by the fallacy of the moon's influence on clouds. 

 The origin of this fallacy is somewhat complicated. In 

 the first place, when the sky is densely clouded the moon 

 would not be visible at all ; it would be necessary for us to 

 see the full moon in order that our attention should be 

 strongly drawn to the fact, and this would happen most 

 often on those nights when the sky is cloudless. Mr. 

 W. Ellis,^ moreover, has ingeniously pointed out that there 

 is a general tendency for clouds to disperse at the com- 

 mencement of night, which is the time when the fuU moon 

 rises. Thus the change of the sky and the rise of the full 

 moon are likely to attract attention mutually, and the 

 coincidence in time suggests the relation of cause and 

 effect. Mr. Ellis proves from the results of observations 

 at the Greenwich Observatory that the moon possesses no 

 appreciable power of the kind supposed, and yet it is 

 remarkable that so sound an observer as Sir John Herschel 

 was convinced of the connection. In his " Eesults of 

 Observations at the Cape of Good Hope," ^ he mentions 

 many evenings when a full moon occurred with a 

 peculiarl}' clear sky. 



1 Philosophical Magazine, 4th Series (1867), vol. xxxiv. p. 64. 



2 See i\'otes to Measures of Double Stars, 1204, 1336, 1477, 1686, 

 1786, 1816, 1835, J9-9> 2081, 2186, pp. 265, &c. See also Herschel's 

 Familiar Lectures on Scientific Subjects, p. 147, and Outlines oj 

 Astronomy, 7lh ed. p. 285. 



