TRANSACTIONS OF THE SECTIONS, 45 



hour by hour, how much dry air passes in towards the centre of the circle through 

 its circumference, and also how much passes out. 



Let us suppose that more is passing in than is passing out, or that the imports 

 into the area of the circle are greater than the exports out of it. Now, the dry 

 air that passes in is incapable of production or of consumption, and hence the 

 stock of the material at the central station, and in the area generally', ought to be 

 on the increase, since we have imagined the imports to be greater than the 

 exports. If, however, we ascertain from actual observation that the stock of dry 

 air is diminishing instead of increasing, we may be sure that some is carried on 

 by an upward current, which of course carries the moisture with the dry air. 



So much for the vertical component ; and in the next place, with regard to the 



S reduction or consumption of aqueous vapour as it passes from place to place. 

 >ur consideration has hitherto been confined to qiiantittj ; let us now define what 

 is meant bj^ the hygrometric quality of the air. It may be represented by the fol- 

 lowing quotient : — 



mass of vapour in a cubic foot 



mass of dry air in a cubic foot 



Now this quotient can only alter by evaporation, by precipitation, or by mixture. 

 This hygrometric quality of the air may perhaps be considered as a quality 

 sufficiently constant to aid us in tracing the actual motion of air, just as we may 

 make use of the element of saltness to trace the actual path of an oceanic cur- 

 rent. But besides this aid, we may make use of it to enable us to tell the precipi- 

 tation or evaporation. For instance, a very damp air, in passing over a very dry 

 country, may be supposed to emerge less damp, having its hygrometric quality 

 changed ; or a very dry air, in passing over a very damp country, may be supposed 

 to emerge less dry, having its quality changed in the opposite direction. Thus, 

 by actual observation of the quality of the air at the tmie of its reaching some 

 particular tract of land or ocean, and at the time of its leaving it, we may pos- 

 sibly get much better observations of what goes on in the country, as far as this 

 particular research is concerned, than if it were studded with gauges. 



I should therefore suggest that meteorological observations should, by a system 

 of reduction, be made to show — 



(1) The mass of dry air and moisture in one cubic foot actually present at 

 each station from hour to hour. 



(2) The mass of dry air and of moisture that passes each station, hour by 

 hour, in two lines of direction at right angles to each other, namely, 

 north and south and east and west. 



When these hom-ly elements are obtained, they might for seasonal changes be 

 reduced after the method of five-day means, or for the investigation of abrupt 

 changes of weather, such as storms, they might be utilized in some other way. 



Iletaining the belief that meteorology ought to be treated as much as possible 

 with the view, in the first place, of determining the actual motions of our atmo- 

 sphere, and, in the next place, of assigning the cause of these, it is no doubt the 

 greater movements of the atmosphere that will be indicated by five-day means. 

 It ought, however, to be remarked that the observations at any station are subject 

 to the influence of locality, none probably more so than those of wind. It would 

 appear that this influence ought to be eliminated before we can make any trust- 

 worthy quantitative deductions regarding the gi-eater movements of our atmosphere. 

 I should, however, imagine that the quality of the air, as herein indicated, may be 

 made of immediate use in the study of storms. 



It has been suggested by Mr. Meldrum, who expresses his concurrence' with 

 the above remarks, that in addition to the five-day means indicated above, there 

 might be given a brief epitome of the weather. Thus, for instance, " The wind 

 blew from the N.E. at Kew from January 1st, 1 a.m., to January 4th, .3 p.m., in all 

 86 hours, at the average velocity of 10 miles an hour, with an average pressure of 

 30 inches, a temperature of 40° F., and an average hygrometric quality repre- 

 sented by -075." The same remarks had previously occurred to myself, and Mr. 

 Aiiy also has recently suggested the study of the meteorological phenomena of 

 those periods during which the wind blows in the same direction. 



