Lieut.- Col. Sabine's Report on the Meteorology of Toronto. 103 



in the indications of tiie barometer, we cannot fail to be immediately struck 

 with the very close correspondence of the diurnal marcli before our eyes with 

 that of the temperature which we have already examined. The maximum 

 of pressure is at 6 a.m.; the minimum at 2 p.m. The progressions take place 

 in the opposite or inverse sense to each other, but they are remarkably har- 

 monious, and leave no doubt of a mutual connexion, and of the dependence 

 either of the one on the other, or of both on a common cause. 



An explanation of this connexion, which presents itself to the mind as soon 

 as the facts are clearly perceived, may be thus stated : — As the temperature of 

 the day increases, the earth becomes warmed and imparts heat to the air in 

 contact with it, and causes it to ascend. The colunm of air over the place of 

 observation thus warmed rises, and a portion of it dift'uses itself, in the higher 

 regions of the atmosphere, over adjacent spaces where the temperature at the 

 surface of the earth is less. Hence the statical pressure of the column is 

 diminished. On the other hand, as the temperature falls, the column con- 

 tracts, and receives in its turn a portion of air which passes over in the higher 

 regions from spaces where a higher temperature prevails ; and thus the sta- 

 tical pressure is augmented. 



This explanation is merely the extension to the particular case of the 

 diurnal variation, of principles which have long been familiar to meteorolo- 

 gists in accounting for various other atmospherical phaenomena, such for 

 example as monsoons, and land and sea breezes. To make the parallel com- 

 plete, it should be shown that, when the temperature rises, an influx of air 

 takes place towards the lower part of the column, proportioned to the ascend- 

 ing current, and tending to replace the air which is thus removed. The obser- 

 vations which will be cited in the sequel of this communication will show 

 that such is precisely the fact at Toronto. The force of the wind, taken 

 without reference to its direction, has also its diurnal variation, corresponding 

 in all respects with the diurnal march of the temperature and of the gaseous 

 pressure ; being a minimum at 6 a.m., and a maximum at 2 p.m. — increasing 

 with the augmentation of the temperature, and decreasing with its diminu- 

 tion. The air which thus flows in, becoming warmed, pursues in its turn the 

 course of the ascending current. We have thus the double evidence of the ex- 

 istence of this current, — 1st, in the diminution of pressure, showing the out- 

 pouring at one extremity ; and 2nd, in the increased force of the wind, showing 

 the inpouring at the other extremity. As the temperature keeps continually 

 rising, both the demand for and the supply of fresh inflowing air progressively 

 increase. The diminution which the gaseous pressure continues to undergo as 

 long as the temperature continues to rise, shows, as we might naturally ex- 

 pect, that the supply is continually somewhat in arrear of the demand. 



The diminution of the gaseous pressure and increase in the force of the 

 wind being consequent on the rise of the temperature, the turning points of 

 the two former phaenomena might be expected to occur somewhat later than 

 the instant of minimum temperature ; and this appears by the tables to be the 

 case, but will probably be more clearly shown M'hen the hourly observations 

 shall come under review. 



Annual Variation. — Let us now proceed to the mean pressure of the 

 gaseous atmosphere in each month of the year, and its consequent annual va- 

 riation. These are shown in the following table : — 



