But the question now arises, as to how this continuous transformation of the axial energy can be consistent with that condition of uniformity of rotation of the planet which was originally assumed. If the total energy of the planetary mass is limited, and if it can receive no increment of energy from any external source, it is clear that the axial energy transformed must, by some process, be continuously returned to its original form. Some process or mechanism is evidently necessary to carry out this operation. This mechanism we conceive to be provided by certain portions of the material of the planet, principally the gaseous matter which resides on its surface, completely enveloping it, and extending outwards into space (§ 38). In other words, the atmosphere of the planet forms the machine or material agency by which this return of the transformed axial energy is carried out. It has already been pointed out (§ 9) how the working energy of every secondary transformation is derived from the original axial energy of the planet itself. Each of these secondary transformations, however, forms but one link of one cyclical chain of secondary transformations, in which a definite quantity of energy, initially in the axial form, passes, in these secondary operations, through various other forms, by different processes and through the medium of different material machines, until it is eventually absorbed into the atmosphere of the planet. These complete series of cyclical operations, by which the various portions of axial energy are carried to the atmosphere, may in some cases be of a very simple nature, and may be continuously repeated over very short intervals of time; in other cases, the cycle may seem obscure and complicated, and its complete operation spread over very long periods, but in all cases the final result is the same. The axial energy abstracted, sooner or later, recurs to the atmospheric machine. By its action in this machine, great masses of gaseous material are elevated from the surface of the planet against the attractive force of gravitation; the energy will thus now appear in the form of potential energy or energy of position. By a subsequent movement of these gaseous masses over the surface of the planet from the regions of high velocity towards the poles, combined with a movement of descent to lower levels, the energy of position with which they were endowed is returned once more in the original axial form.