THE OBSERVATIONS OF AIR AND SEA MOTIONS. 



15 



We emphasize this as a method which may be seriously thought of, especially 

 later, when more complete observations can be obtained. But for the first attempts 

 we shall prefer a more summary method, using the same sheets which we have 

 already introduced in statics for the representation of the fields of pressure and of 

 mass. This will be convenient for several reasons. First, the kinematic diagnosis 

 is not complete as long as we know only the velocities. We must also know the 

 amounts of mass which have these velocities. Using the sheets introduced in 



4400- 



Fig. 33. Construction of average velocities for standard level sheets. 



statics, we get a coherent representation of velocities and of masses. Further, our 

 final aim being the performance of dynamic investigations, we shall arrange every- 

 thing convenient for future purposes by choosing our representation of the field of 

 motion in as close connection as possible with that of the field of pressure. We shall 

 therefore use either level sheets of the thickness of 1 ,000 dynamic meters, or isobaric 

 sheets corresponding to the difference of pressure of ioom-bars. In so doing, we 

 shall evidently often get sheets which are too thick for a detailed representation of 

 the motions. But the way of refining the representation by the choice of thinner 



