546 KEPORT— 1903. 



actually obeys, but which enabled Newton to deduce the law of gravitation. The 

 first great step in the development of any physical science is to substitute for 

 the indescribably complex reality of nature an ideal system that is an effective 

 equivalent for the purposes of theoretical computation. I cannot refrain from 

 quoting again from Plato's ' Eepublic ' a passage which I have quoted elsewhere 

 before. It expresses paradoxically but still clearly the relation of natural philo- 

 sophy to natural science. In the discussion of the proper means of studying 

 sciences Socrates is made to say, ' We shall pursue astronomy with the help of 

 problems just as we pursue geometry ; but we shall let the heavenly bodies alone 

 if it is our design to become really acquainted with astronomy.' What I take to 

 be the same idea is expressed in other words by Rayleigh in the introduction to 

 his ' Sound.' He there points out as an example that the natural problem of a 

 sounding tuning-fork really comprises the motion of the fork, the air, and the 

 vibrating parts of the ear ; and the first step in sound is to simplify the complex 

 system of nature by assuming that the vibrations of the fork, the air, and the ear 

 can be treated independently. In many sciences this step is a most diificult one to 

 take. What student of nature, contemplating the infinity of heavenly bodies and 

 unfamiliar with this method of idealism, would imagine that the most remarkable 

 and universal generalisation in physical science was arrived at by reducing the 

 dynamics of the universe to the problem of three bodies ? When we look round 

 the sciences each has its own peculiar ideals and its own physical quantities : 

 astronomy has its orbits and its momentum, sound its longitudinal vibration, light 

 its transverse vibration, heat its energy and entropy, electricity its ' quantity ' and 

 its wave, but meteorology has not yet found a satisfactory ideal problem to substi- 

 tute for the complexity of nature. 1 wish to consider the aspect of the science 

 from this point of view and to recall some of the attempts made to arrive at a 

 satisfactory modification of reality. I do not wish to refer to such special applica- 

 tions of physical reasoning as may be involved in the formation of cloud, the 

 thermodynamics of a mixture of air and water vapour, the explanation of optical 

 or electrical phenomena, nor even Helmholtz's application of the theory of gravi- 

 tational waves to superposed layers of air of different density. These require 

 only conventions which belong already to physics, and though they may furnish 

 suggestions they do not themselves constitute a general meteorological theory. 



The most direct efforts to create a general theory of atmospheric circulation are 

 those which attempt to apply Newtonian dynamics, with its more recent develop- 

 ments on the lines of hydrodynamics and thermodynamics. Attempts have been 

 made, mathematical or otherwise, to determine the general circulation of the 

 atmosphere by the application of some form of calculation, assuming only the 

 sun and a rotating earth, with an atmosphere, as the data of the problem. I con- 

 fess that these attempts, interesting and ingenious as they are, seem to me to be 

 somewhat premature. The 'problem' is not sufficiently formulated. When 

 Newton set to work to connect the motions of the heavenly bodies with 

 their causes, he knew what the motions of the heavenly bodies were. Mathe- 

 matics is an excellent engine for explaining and confirming what you know. It 

 is very rarely a substitute for observation, and before we rely upon it for telling us 

 what the nature of the general circulation of the atmosphere really is, it would 

 be desirable to find out by observation or experiment what dynamical and elastic 

 properties must be attributed to an extremely thin sheet of compressible fluid 

 rotating about an axis with a velocity reaching 1,000 miles an hour, and subject to 

 periodic heating and cooling of a very complicated character. It would be more iu 

 consonance with the practice of other sciences to find out by observation what the 

 general circulation is before using mathematics to explain it. What strikes one most 

 about the mathematical treatises on the general circulation of the atmosphere ia 

 that what is true about the conclusions is what was previously known from 

 observation. It is, I think, clear that that method has not given us the working 

 idfeal upon which to base our theory. 



Consider next the attempts to regard atmospheric phenomena as periodic. Let 

 me include with this the correlation of groups of atmospheric phenomena with each 

 other or with those of the sun, when the periodicity is not necessarily regular, and 



