METEOROLOGICAL REGISTER KEPT AT LE1TH FORT IN 1826 AND 1827. 359 





xioius oi moimug auu rjvemu 

 Mean Temperature. 



3 Critical Interval 



In 1824 it occurred at 



f 9»» 13"> A.M. ) 



• \ 8 26 p.m. j" 



Hh 13m 



In 1825, 



J 9 13 A.M. \ 



' (8 28 p.m. j 



11 15 



In 1826, 



J 9 7 A.M. ) 

 • |8 27 p.m.} 



11 20 



In 1827, 



J 9 12 A.M. ) 



• (8 23 p.m./ 



11 11 



Mean, ll h 15 m 



The mean of which is 



9 h ll m a.m. and 8 h 26 m p.m. 



The interval between the morning and evening mean temperature has been called 

 the criticalinterval, which at Leith Fort is ll h 15 m , and which, there is reason to 

 believe, is a fixed quantity. The equality of these numbers in four different years 

 is very remarkable, the deviation of each from the mean not exceeding 4 m . 



Although the hours of mean temperature vary in different latitudes, and at 

 different heights above the sea, yet the critical interval seems to be a fixed 

 quantity everywhere, as appears from the following table : — 



At Padua, . 



ll h 



14m 



At Philadelphia, . 



ll h 



20' 



At Appenrade, 



11 



14 



At Belleville, 



11 



14 



At Inverness, 



11 



13 



At Trincomalee, 



11 



5 



At Tweedsmuir, 



11 



15 



At Kingussie, 



10 



44 



The mean of which is IP 10 m , differing only 4 m from the Leith result. 



The determination of the times of mean annual temperature gives us the two 

 best hours for recording the indications of the thermometer, namely, 9 h ll m a.m. 

 and 8 h 26 m p.m. ; for if any of the observations is accidentally omitted at one of the 

 hours, the mean of the remainder will approach nearer to the mean temperature 

 of the year than if any other pair of hours had been taken and similar omissions 

 made. 



Another advantage of this determination is, that the mean temperature of the 

 year may be obtained with great accuracy from a single observation made every 

 day at one of the hours of mean temperature. 



If we examine the annual, or even the monthly, curves, it will be seen that the 

 ascending, or morning branch, is more regular in its progression than the descend- 

 ing, or evening branch, and therefore a single observation made at the time 

 of the morning mean is preferable to one made at the time of the evening 

 mean. 



This regularity in the morning curve has been observed in other phenomena, 

 but especially in atmospherical polarisation, and the cause of it has been explained 

 by Dove and Rubenson* 



* Memoire sur la Polarisation de la Lumiere Atmospherique, p. 86, note. 

 VOL. XXIV. PAET II. . 5 E 



