xviii.] OBSERVATION. 411 



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 interfiltration 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 ignorant of their 

 occurrence. 1 The highest strata of the atmosphere are 

 wholly imperceptible to us, except when rendered luminous 

 by aurora] currents of electricity, or by the passage of 

 meteoric stones. Most of the visible phenomena of comets 

 probably arise from some substance which, existing pre- 

 viously invisible, becomes condensed or electrified suddenly 

 into a visible form. Sir John Herschel attempted to 

 explain the production of comet tails in this manner by 

 evaporation and condensation. 2 



Negative Arguments from Non-observation. 



From what has been suggested in preceding sections, it 

 will plainly appear that the non-observation of a pheno- 

 menon is not generally to be taken as proving its non- 

 occurrence. As there are sounds which we cannot hear, 

 rays of heat which we cannot feel, multitudes of worlds 

 which we cannot see, and myriads of minute organisms 

 of which not the most powerful microscope can give us 

 a view, we must as a general rule interpret our experience 

 in an affirmative sense only. Accordingly when inferences 

 have been drawn from the non- occurrence of particular 

 facts or objects, more extended and careful examination 

 has often proved their falsity. Not many years since it 

 was quite a well credited conclusion in geology that no 

 remains of man were found in connection with those of 



1 Jevons, On the Cirrous Form of Cloud, Philosophical Magazine, 

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



2 Astronomy, 4th ed. p. 358 



