i8 



DYNAMIC METEOROLOGY AND HYDROGRAPHY. 



by the specific volumes (column 3). The result is given in column 6. But as 

 in the preceding case, we have to remark that the values of specific momentum 

 found in this way are not the exact height-averages of specific momentum for these 

 sheets; yet they give as a rule sufficient approximation toward these averages. If 

 the exact values are required, we have to change the velocities given in table A into 

 specific momenta before performing the construction leading to the averages. 



Table C. Average horizontal motion in standard isobaric sheets. Pavia (lal. 45 11', 

 long. qio'E.), July 25, 1907, 7*33" 7*48 Greenwich. 



106. Special Remarks. As we see, the result contained in columns 4 and 5 

 of table B, i.e., the average velocities of the air in the level sheets, can be found 

 without any knowledge of the registered values of pressure, temperature, and 

 humidity. For everything regarding specific momenta or motions in isobaric 

 sheets, a certain knowledge of the fields of pressure and of mass is required. But on 

 account of the very limited accuracy of the observations of velocities we shall never, 

 for the purely kinematic purposes, need to know these fields of pressure and of mass 

 with the accuracy used in statics for the purpose of drawing their synoptical repre- 

 sentations. Pressure and temperature being observed near the ground, and the 

 values of the temperature being estimated for greater heights, it will be easy, by 

 using our meteorological tables (iim, 15M, 12M, iom), to find the heights of the 

 standard isobaric surfaces or the pressures in standard levels and the correlated 

 data regarding specific volume or density, with sufficient accuracy for kinematic 

 purposes. 



It is further worth while mentioning that the result of an ascent (as contained, 

 for instance, in columns 4 and 5 of table C), may be condensed into a telegram 

 which could be sent to a central office. (Compare Statics, section 57.) While two 

 men are taking the observations, a third can perform the constructions and the 

 calculations, and it will be possible to send off the telegram a few minutes after the 

 last observation of the pilot-balloon is taken. Thus there will be no technical 

 difficulty in bringing observations of this kind into application for the daily weather 



service. 



107. Main Example: Europe, July 25, 1907. The most complete observations 

 which we have had at our disposal for working out a diagnosis of atmospheric 

 motions are those obtained on July 25, 1907. This day belonged to a period of 



