METEOROLOGICAL OBSERVATIONS AT PLYMOUTH. 157 



Table XL 



Showing the Mean Morning and Evening Oscillations of Spring, 

 Summer, Autumn and Winter, in Periods of three Months 

 each, with their Differences from the Mean Oscillation of 

 each Season, &c., of the Year ; also the Difference of the 

 Morning and Evening Mean Oscillations. 



Seasons. 



il 



So 



A.M. 



P.M. 



in 

 III 



5S 



§ 



1 



O 



Differences. 



§ 



1 



Differences. 



Season. 



Year. 



Season. 



Year. 



Spring ... 

 Summer . 

 Autumn . 

 Winter... 



•0170 

 •0130 

 •0185 

 •0230 



•0120 

 •0095 

 •0175 

 •0245 



•005- 

 •003- 

 ■001- 

 •0015+ 



•0048- 

 •0073- 

 •0006+ 

 •0077+ 



•0220 

 •0165 

 •0195 

 •0215 



•005+ 

 •003+ 

 •001 + 

 •0015- 



•005+ 

 •0003- 



•0026+ 

 •0047+ 



•010 

 •007 

 •002 

 •003 



16. The differences in the amount of the morning and evening 

 waves appear to be so regularly developed, that I cannot doubt 

 of its being included in some general law of the daily pressure. 

 The fact receives further confirmation from the observations 

 recorded by Lieut.-Col. Sykes, in his valuable paper on the 

 Meteorology of Dukhun*, by which it appears that the amount 

 of the ascent of the barometer, between 4 p.m. and 10 p.m., is 

 generally greater than that between 4 a.m. and 10 a.m. 



I find however by these observations, that the amount of the 

 wave is increased at night rather by the previous depression 

 than by the following maximum, since the pressure has been 

 seldom observed to attain so high a point at night as that from 

 which it descended in the morning. In our observations, how- 

 ever, the maximum at 10 p.m. is generally greater, by a small 

 quantity, than the maximum at 10 a.m., except in the three 

 months of winter already mentioned. The accuracy of this re- 

 sult we have no reason to doubt ; for, besides that the obser- 

 vations have been reduced to 32°, the temperature of the re- 

 spective times of observation, viz. 10 a.m. and 10 p.m., did not 

 differ by above '4 of a degree. 



17. It may appear, however, worth while to consider how far a 

 difference of temperature at 4 a.m. and 4 p.m. may, by operat- 

 ing on the mercurial vapour, cause an undue depression of the 

 column at the latter hour ; but this source of error is decidedly 

 negatived by the results given in Table II., Plate II., for the 

 months of summer, in which we find the amount of depression 

 at 4-5 A.M. greater than at 4-5 p.m. I am content therefore, at 



* Philosophical Transactions for 1835. 



