426 Mr. W. S. Jevons on the Deficiency of Rain in an 



wind, but might perhaps be explained by comparison with the 

 direction and force of the wind combined. They serve me 

 here amply to establish the unsatisfactory nature of the best 

 rain measurements. 



13. It is in this subject quite fallacious to appeal to average 

 results ; for an appearance of uniformity and law will arise in the 

 long run, according to the doctrine of probabilities, however 

 irregular and various the causes which produce the difference. 

 A law of nature must appear in every case in which it acts alone, 

 reasonable error of observation being allowed for ; but the dis- 

 crepancies of individual rain observations at different altitudes 

 are such as can come under no law. Even average or total 

 quantities for short periods are extremely discordant. Prof. 

 Phillips's observations are stated in weekly totals*; but in the 

 week February 19 to 26, 1832, we find that the lowest gauge 

 received nearly six times as much rain as the upper one upon 

 the York Minster, while in the next succeeding week but one 

 the lower gauge contained only 1*22 times (or lj) as much 

 as the other. The circumstances fully explain this difference, 

 "violent gales" having occurred in the former week and "per- 

 pendicular rain, without a trace of wind, in large drops " in the 

 latter. This last statement will again be referred to (see par. 27). 



14. Arago's results at the Paris Observatory, although pretty 

 uniform when stated in yearly averages, exhibit similar dis- 

 cordances in the separate months. From a Table in the Ency- 

 clopedia Metropolitana, Art. Meteorology (p. 115), I extract the 

 results of the following three months, being the 



Difference in Centimetres between results of Higher and Lower 

 Gauges at the Paris Observatory. 



1826. 1827. 1828. 1829. 



March . . . -285 1-207 '790 -174 



May .... -430 1-575 -210 -010 



December . . -810 1-220 -190 -030 



Here in the same month, May, the difference varies from y^ to 



im> (centimetre) ! 



1 5. The deficiency of rain inan elevated rain-gauge varies greatly 

 according to the season of the year; and ona n average the greatest 

 deficiency is found during the winter. It is a phenomenon of a 

 ivintry character, observes Prof. Phillips f. But of all the months 

 March generally shows the largest deficiency J; and Prof. Phillips, 



* Brit. Assoc. Report, 1833, Trans. Sections, p. 403. 



t Brit. Assoc. Report, 1834, Trans. Sections, p. 562. See also Howard's 

 ' Climate of London,' vol. i. p. 104 ; and Schouw, Climat d'ltalie, p. 135 : 

 " qu'elle est beaucoup plus forte en hiver qu'en ete." 



X See the observations of Dr. Heberden, Phil. Trans, vol. lix. (1769) 



