4 DYNAMIC METEOROLOGY AND HYDROGRAPHY. 



We shall call this the dynamic method; for in virtue of the laws of hydro- 

 dynamics and thermodynamics which govern atmospheric or hydrospheric phe- 

 nomena, preceding states are in relation of causality to subsequent states. Inasmuch 

 as we know the laws of hydrodymanics and thermodynamics, we know the intrinsic 

 laws according to which the subsequent states develop out of the preceding ones. 

 We are therefore entitled to consider the ultimate problem of meteorological and 

 hydrographic science, that of the precalculation of future states, as one of which 

 we already possess the implicit solution, and we have full reason to believe that we 

 shall succeed in making this solution an explicit one according as we succeed in 

 finding the methods of making full practical use of the laws of hydrodynamics and 

 thermodynamics. 



91. Three Partial Problems. Evidently general investigations according to 

 the dynamic plan must lead to occupation with three special problems. The first 

 is the question of the organization of observations serving these investigations. The 

 observations being given, the next problem will be to work out from them synoptical 

 representations of the fields serving to define actual states of atmosphere or hydro- 

 sphere. Introducing a terminology taken from medical science, we shall call this the 

 problem of diagnosis of atmospheric or hydrospheric states. The result of a diagnosis 

 being given, the final problem will be that of precalculating future states. Making 

 continued use of the same terminology, we shall call this the problem of prognosis 

 of future states. Before returning to details, we shall make some general remarks 

 on each of these three problems, taking as the leading idea that the condition for 

 real progress is to arrange so that full use can be made of the knowledge contained 

 in the laws of hydrodynamics and thermodynamics. 



92. Principles for the Organization of Observations. It is of course not 

 possible to know how observations will be organized later, when the problems of 

 diagnosis and of prognosis are completely solved in explicit form. But the question 

 interesting the present generation of investigators is to get that organization which 

 would facilitate as much as possible the work with the solution of these problems. 



From what we have evolved already it will be clear that the dynamic method 

 requires simultaneous observations. The principle of simultaneity being therefore 

 agreed upon as the fundamental one, the next questions will be those of the distribu- 

 tion in space of each set of simultaneous observations and the distribution in time of 

 the successive epochs of observations. 



In order to answer these questions, we have to remark that the fundamental 

 laws of hydrodynamics and thermodynamics have the form of partial differential 

 equations giving relations between the continuous space- variations and time-varia- 

 tions of the different elements. To make it as easy as possible to bring them into 

 application, we must try to organize observations so as to realize an approximation 

 toward continuity in space and time. In other words, the distances in space 

 between the points of observation and the distances in time between the epochs of 

 observation must be small enough to be used, with a certain degree of approxima- 

 tion, as line-differentials and time-differentials. 



