SUPERIOR REGIONS OF THE EARTH S ATMOSPHERE. 399 



take those variations into account in the calculation of the tables. The 

 next thing to be done would be to observe the hygrometrical state of 

 the inferior layers, and in particular to measure the amount of its di- 

 minution in proportion to the height to which we ascend; for these 

 elements also, though in a very inferior degree, affect the same con- 

 stants. The real constitution of the atmosphere up to very consider- 

 able degrees of elevation being known, the methods which I have 

 given in my Memoir will therefore enable us to deduce the refraction 

 numerically for the layers to which they are applied. When we have 

 arrived at elevations at which the valuation by limits becomes sufficient, 

 the remainder of the refraction will then be obtained by this process 

 without any hypothesis. We shall then have tables of refraction 

 adapted to circumstances which are really variable, though supposed 

 in the present tables to be uniform. And, should we be thus led to 

 discover, (a thing not at all improbable,) that these variations, or at 

 least the most considerable of them, take place chiefly in those atmo- 

 spheric layers which are not at a great elevation, it might be possible 

 in the great observatories of Europe to observe regularly the constants 

 of those troubled regions of the air by means of small captive balloons, 

 carrying with them instruments with indicators, and the results obtained 

 in this way might be applied as correctives to permanent tables con- 

 structed for the untroubled region. The only errors then to be appre- 

 hended would be those which might arise from an accidental alteration 

 in the supposed sphericity of the refringent layers; such an alteration, 

 for instance, as might be produced by a violent agitation continued for 

 a long time in one direction. But the effects of these disturbances 

 being thus isolated, and their extent being known, it would perhaps 

 be not impossible to give them due attention, if they should be found 

 to possess any constancy, and analysis would have then done for the 

 theory of astronomical refractions all that it is allowed us to expect. 



