NOTES. 



rarefied, the column, extending in height, overflows laterally in the higher 

 regions of the atmosphere, and the statical pressure at the base of the column 

 is diminished. In the afternoon the converse takes place : the column of air 

 cools, condenses, and, contracting in height, receives the overflow from the 

 adjacent air, which in its turn becomes heated ; and thus the statical pressure 

 is increased. The stations at which this more simple form of the barometric 

 diurnal curve has hitherto been found to take place are Catherinenbourg, 

 Nertchinsk, and Baruaoul, all in the Russian territories, on the confines of 

 Em-ope and Asia. 



"Whenever a free communication with a sufficiently extensive evaporating 

 surface exists, a diurnal variation is also found to take place in the tension 

 of the aqueous vapour in the atmosphere, proportioned in amount to the 

 diurnal variation of the temperature. If the sources of vapour be ample to 

 supply the drain of the ascending current of the air which carries vapour 

 with it, as well as to furnish the increasing tension which the increasing 

 temperature demands, the diurnal variation of the vapour tension has its 

 maximum at or near the hottest hour of the day, and its minimum at or near 

 the coldest hour, being the converse of the diurnal variation of the dry air (or 

 of the gaseous portion of the atmosphere), which, as before stated, has its 

 maximum at or near the coldest hour, and its minimum at or near the warmest 

 hour. The combination of the diurnal variations of the vapour and of the dry 

 air, which conjointly produce the diurnal variation in the pressure on the ba- 

 rometer, occasions the double maximum and minimum of the barometric curve 

 in the temperate zone, at stations not so far removed as the Russian ones from 

 the sources from whence vapour can be supplied. If the elastic force of the 

 vapour be observed by means of an hygrometer with the same care that the 

 barometer is observed, and if the respective pressures of the elastic forces of 

 the air and of the vapour upon the mercury of the barometer be separated 

 from each other, the diurnal variation of the dry air exhibits at all stations in 

 the temperate zone at which observations have hitherto been made, a similar 

 curve to that which the whole barometric pressure produces at the Russian 

 stations where the air is naturally dry. 



At Prague in the interior of Europe, at Toronto in the interior of America 

 but situated near extensive, lakes, and at Greenwich in the vicinity of the 

 ocean, the diurnal variation of the dry air has but one maximum and one 

 minimum, and these coincide, or nearly coincide, with the coldest and the 

 warmest hours (Sabine, Reports of the Brit. Assoc. 1844, pp. 4262). 

 Hence it has been inferred, that the normal state of the diurnal variation of 



