OBSERVATION. 15 



pointed out that there is a general tendency for clouds 

 to disperse at the commencement of night, which is the 

 time when the full moon rises. Thus the change of the 

 sky arid 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 acute and 

 sound an observer as the late Sir John Herschel was 

 convinced of the connection. In his ' Results of Obser- 

 vations at the Cape of Good Hope'P, he mentions many 

 evenings when a full moon occurred with a peculiarly 

 clear sky. 



There is yet a fourth class of cases, in which B is really 

 the antecedent event, but our perception of A, which is a 

 consequence of B, may be necessary to bring about our 

 perception of B. There can be no doubt, for instance, 

 that upward and downward currents are continually cir- 

 culating in the lowest stratum of the atmosphere during 

 the day-time ; but owing to the transparency of the at- 

 mosphere we have no evidence of their existence until we 

 perceive cumulous clouds, which are the consequence of 

 such currents. In like manner an intern* Itratioii of bodies 

 of air in the higher parts of the atmosphere is probably in 

 nearly constant progress, but unless threads of cirrous 

 cloud indicate these motions we remain wholly ignorant of 

 their occurrence 1. The highest strata of the atmosphere 

 are wholly imperceptible to us, except when rendered 



P See. ( Notes to Measures of Double Stars/ 1204, 1336, 1477, 

 1686, 1786, 1816, 1835, 1929, 2081, 2186, pp. 265, &c. See also 

 Herschel's 'Familiar Lectures on Scientific Subjects, p. 147, and 'Out- 

 lines of Astronomy,' 7th ed. p. 285. 



q Jevons, 'On the Cirrous Form of Cloud,' Philosophical Magazine, 

 July, 1857, 4th Series, vol. xiv. p. 22. 



