26 Lieut.- Col. Sabine on some Points in 



the periodical variations in the temperate zone, and into the se- 

 quence of natural causes and effects, in accordance with which the 

 annual and diurnal fluctuations of the elastic forces of air and vapour 

 at the surface of the earth depend on the variations of temperature : 

 and from these premises it was inferred, that the normal state of the 

 diurnal variations of the pressures of the air and vapour and of the 

 force of the wind, in the temperate zone, might be regarded as that 

 of a single progression with one maximum and one minimum, the 

 epochs of which should nearly coincide with those of the maximum 

 and minimum of temperature *. 



That exceptions should be found to this state of things in par- 

 ticular localities in the temperate zone was far from being impro- 



* Since this communication was read at Cambridge I have received from M. 

 Dove a copy of a paper read to the Academy of Berlin, entitled ' Ueber die perio- 

 dischen Aenderungen des Druckes der Atmosphare im Innern der Continente,' in 

 which the remarkable facts are stated, that at Catherinenbourg and Nertchinsk (on 

 the mean of several years), and at Barnaoul (in the years 1838 and 1840), the 

 mean diurnal barometric curve itself exhibits but one maximum and one mini- 

 mum in the twenty-four hours ; the maximum coinciding nearly with the coldest, 

 and the minimum with the hottest hours of the day. At these stations therefore, 

 and in the years referred to, the forenoon maximum disappeared, and the barome- 

 tric curve assimilated in character to the curve of the dry air in other places in 

 the temperate zone. These stations are situated far in the interior of the greatest 

 extent of dry land on the surface of our globe, and at a very great distance from an 

 expanse of water, from whence vapour can be supplied. The diminished pressure 

 of the dry air produced by the ascending current and overflow as the temperature 

 of the day increases, is not therefore compensated by an increased elasticity of 

 vapour, and the curve of the diurnal variation of the barometer approximates to 

 the form assumed when the elasticities of the vapour at the several hours of ob- 

 servation are abstracted. This assimilation in character of the barometric and 

 (inferred) gaseous curves, which is thus found to take place in cases where, from 

 natural causes, the influence of the vapour is greatly lessened, appears a confir- 

 mation of the propriety of separating the effects of the elastic forces of the dry 

 air and vapour in their action on the barometer. 



M. Dove considers that the single progression of the diurnal barometric curve, 

 which takes place at the three Asiatic stations referred to in this note, is character- 

 istic of a true continental climate. It is, without doubt, characteristic of an ex- 

 treme climate, and as such is highly instructive. There appears reason to doubt 

 whether an extreme climate of corresponding character exist at all in the tempe- 

 rate latitudes of the continent of America. 



If, however, we examine the record of the observations made hourly in the year 

 1842 at Catherinenbourg, Barnaoul and Nertchinsk, in the • Annuaire Magnetique 

 et Meteorologique de Russie,' we find that at Catherinenbourg in that year the ba- 

 rometer exhibits a double progression, but that the morning maximum, which oc- 

 curs at the observation hour of 8 h 22 m a.m., exceeds the antecedent minimum only 

 by a quantity less than 0003 in. At Barnaoul there is also a double progression 

 in the barometric mean in that year, the morning maximum being still small, and 

 taking place between the observation hours of 9 b 54 m and 10 h 54 m a.m. At Ner- 

 tchinsk also there is a morning maximum occurring at the observation hour of 9 h 

 17 m a.m. In all the three cases the double progression shown by the barometer 

 disappears wholly in the curve of the dry air, which curve exhibits at these three 

 stations, as well as at Toronto, Prague and Greenwich, but one maximum and one 

 minimum in the twenty-four hours. At the three stations of extreme dryness cited 

 by M. Dove, therefore the vapour was still sufficient to impart, in the year 1842 

 at least, a double progression to the diurnal variation of the barometer ; but the 

 hour of the morning maximum was earlier than where the increase of vapour, as 

 the day advances, is greater. 



