NOTES. Ci 



the gaseous portion of the atmosphere in the temperate zone is that of a single 

 progression, having a maximum at or near the coldest hour of the day, and a 

 minimum at or near the warmest hour ; that at stations of remarkable natural 

 dryness, this curve is given directly by the barometer ; and that at other 

 stations, where aqueous vapour mixes in the atmosphere in a greater degree, 

 the same curve is deducible from that of the barometer, by separating 'the 

 elastic force of the vapour from the whole pressure shewn by that in- 

 etrument. 



As a concomitant phenomenon with the diurnal variation of the gaseous 

 pressure, and due originally to the same cause, (viz. the alternate heating and 

 cooling of the earth's surface by radiation, although an immediate consequence 

 of the ascending current,) we find a diurnal variation taking place in the force 

 of the wind at or near the base of the column. At Greenwich and Toronto, 

 where the force of the wind is continually recorded by self-registering instru- 

 ments, it is found to undergo a diurnal variation consisting of a single progres- 

 sion, having a minimum at or near the coldest hour, and a maximum at or 

 near the warmest hour of the day. Soon after the ascending current haa 

 commenced, a lateral influx is produced to supply the drain which it occasions ; 

 the cooler air as it arrives becomes heated in its turn and ascends ; as the 

 upward current gathers strength with the increasing temperature of the day, 

 the lateral influx augments also, and attains its maximum about the hour when 

 the phenomena of increasing temperature give place generally to those of 

 decreasing temperature. 



The insight which has been obtained into the mutual relations of thes 

 meteorological phenomena, and into the sequence of natural causes and effect! 

 by which their connection is explained, is a further illustration of the progresi 

 which is made in the physical sciences by the aid of mean numerical values. 

 The connection which the knowledge of these values has established between 

 the diurnal phenomena of the different elements, and the dependence which 

 it has shewn them to have on a common cause (viz. on the rotation of the 

 earth on its axis, whereby each portion of its surface is successively turned 

 towards the sun, and each meteorological element is thus subjected to a fluc- 

 tuation of which the period is measured by a day), adds another beautiful 

 instance to those which have been cited by M. de Humboldt in the text, of 

 the simplicity with which general results in the physical sciences can be pre- 

 sented, when the links of mutual relation are discovered between phsenomena, 

 which, when looked at singly and superficially, appear unconnected, and when 



