J'.iii Dr. C. Johnston-- Si..n,.y. 



Now, none of the gases of the atmosphere have even approached 

 any such state. Changes incessantly go on in the open air at the 

 bottom of the atmosphere, and the extent and abruptness of the 

 changes that as incessantly go on in its upper regions are probably 

 greater. 



Again, the consequences of cumulative effects arising in the illimit- 

 able trains and combinations of encounters that are taking place, and 

 of associated events in the aither, will also need to lie either explicitly 

 or implicitly taken into account in any valid investigation of the 

 escape of gases from atmospheres by the deductive method. 



All the circumstances that have been referred to would have to 

 appear among the data of an ordinary dynamical investigation of the 

 escape of an individual molecule from an atmosphere, if such an 

 investigation were possible ; and the claim of a generalised theorem 

 like that of the partition of energy to render it unnecessary to go into 

 these details, ought to be carefully scrutinised. In one case at least 

 the claim does not appear to stand this test, viz., in reference to the 

 supposed legitimacy of the assumption that the field of force surround- 

 ing the earth is constant. Though its variations are minute they are 

 none the less real, and are due to interactions between each gaseous 

 molecule and all the molecules of the solid earth, as real as are the 

 interactions between gaseous molecules when they encounter, and as 

 much entitled to be taken into account, when we seek to carry on the 

 investigation in the region of generalised propositions. It should be 

 kept in mind that in reference to what happens within this region, 

 the plea of l>eing so minute as to be of negligible amount is not 

 admissible. Whether a very small factor may or may not be neglected 

 must be determined independently in each individual case ; and in the 

 above instance the decision is that it may not be neglected. 



Other corrections might be suggested along with the principal ones 

 noticed above that relating to the two kinds of temperature, that 

 relating to the field of force, and that relating to turmoil in the atmo- 

 sphere ; but what seems most to be wanted is that we should recognise 

 that any law for a distribution of energy within the atmosphere by 

 itself, can only come approximately into practical effect after the lapse 

 of a sufficient duration, and throughout a column of the atmosphere 

 from which accidents are excluded ; and that this law will not be the 

 Bolt/mann-Maxwell law, which may not be so restricted. 



Thus, let us imagine a cylinder like a great Tower of Babel, reaching 

 to the top of the atmosphere, with walls competent to intercept dynami- 

 cal, electrical and all other extraneous influences other than gravitation. 

 The air within this tube would consist of molecules, moving in a field 

 of force caused mainly by the earth's attraction and rotation, and this 

 column of air might perhaps after some such period as a month, a year, 

 a century, or a thousand years nearly attain such a distribution of 



