44 REPORT — 1869. 



With respect to the motion of our atmosphere, it cannot be anticipated that 

 we shall ever possess the same sort of knowledge which astronomy gives us regard- 

 ing the motions of the heavenly bodies ; for in the latter case the identity of the 

 object is not lost sight of, while in the former case it is clearly impossible to ascer- 

 tain the motions of individual particles of air. Our inquiries into the distribution 

 and motion of the elements of our atmosphere must therefore be pursued by that 

 method which enables us to ascertain the distribution and motion of any other 

 substance or product with the individual components of which we find it imprac- 

 ticable to deal. 



Suppose, for instance, we wish to ascertain the wealth of our country in grain 

 or in spirits, and the distribution of this commodity over the earth's surface. We 

 should first of all begin by taking the stock of the commodity corresponding to a' 

 given date ; we should next keep a strict account of all the imports and exports of 

 the material, as well as of its home production and home consumption. 



Now, if we have taken stock properly at first, and if our account of the imports, 

 the exports, the production, and the consumption of om* material is accurate and 

 properly kept, it will obviously be unnecessary to take stock a second time. But 

 if these accounts are not kept with sufficient accuracy, or if we suspect that om* 

 material leaves us by some secret channel which we wish to trace, it will clearly 

 be necessary to take stock frequently ; and thus a comparison of our various 

 accounts may enable us to detect the place and circumstances of that secret transit 

 which has hitherto escaped our observation. 



Applying these principles to the vapour of our atmosphere, what we wish to 

 know is the amount of the material present at any one station at anj' moment, and 

 also the laws of its motion. It would appear that the best way of measuring the 

 amount present at any moment is by ascertaining the 7nass of vapour present in a 

 cubic foot of air, mass and "s-olume being fundamental physical conceptions. 



Next, with regard to the motion of the atmosphere, including its vaporous con- 

 stituent, the method of coordinates suggested by Dr. Robinson would appear to 

 be the natural way of arriving at this. Let us set up at a station two imaginary 

 apertures, one facing north and south and the other east and west, and gauge the 

 mass of dry air and the mass of moisture that passes each of these openings in one 

 hour ; we shall by this means get the nearest attainable approach to the elements 

 of motion of the atmospheric constituents from hour to hour. We shall not, how- 

 ever, obtain by this means a complete account of this motion, for we have at pre- 

 sent no means of measuring its vertical component. This vertical component 

 corresponds in fact to the secret channel in the illustration given above, which we 

 must endeavour to detect by some indirect method. Another thing that ought to 

 be determined is the production or consumption of the vaporous element of our 

 atmosphere as it passes from place to place. This might be done could we keep 

 an accurate account of the evaporation and the precipitation, the two processes 

 by which this element is recruited and consumed. This would, however, be a 

 very difficult observation. 



Let us now recapitulate what information regarding moisture we can obtain 

 from such meteorological observations as are at present made. We have — 



(1) The mass of vapour actually present at a station from hour to hour. 



(2) The mass that passes a station in one hour, going east and west. 

 (.3) The mass that passes a station in one hom-, going north and south. 



There is wanting — 



(4) The vertical component of the motion of vapour. 



(•5) Its production or consumption as it passes from place to place. 



These deficiencies may, however, be to some extent overcome by the following 

 considerations : — • 



First, the atmosphere moves as a whole when it moves, the dry and moist air 

 moving together ; secondly, dry air is neither capable of production or of consump- 

 tion, but always remains constant in amount. 



To illustrate this part of the snbject, let it be supposed we wish to investigate 

 the vertical motion of the atmosphere at a certain station. Make this station the 

 imaginary centre of a circle, the circumference of which may be supposed to be 

 studded with other stations at sufficiently frequent intervals, so that we can tell, 



