282 



DEPARTMEXT OF THE XATAL SERTICE 



movement, have but a single direction of movement, it follows that the lines of cur- 

 rent for surface water must answer to the equation 



d y 



= f i,ry) 



where x an y are the geographical co-ordinates of the sea's surface. For 



sin {ax + by) 

 we obtain the courses of the current shown in fig. 57. 



d y 

 d X 



Fig. 57. — Oscillation of surface water, in area with water layers in stability. 



The existing material of simultaneous measurements of velocity in the sea is unfor- 

 tunately too small to permit of our verifying the current course here obtained by 

 mathematical means. The meteorology is in this respect far more favourably situated, 

 owing to the large number of stations from which the direction of the wind' is obser- 

 ved simultaneously. And it is then a very simple matter to discover the existence of 

 such oscillations. Figs. 58 and 59 serve to illustrate this. Fig. 58 shows direction 

 and force of the wind, with distribution of atmospheric pressure on January 7, 19€2, 

 at 9 p.m. At a first glance, the direction of the wind would appear to be highly irre- 

 gular, not only in comparison between the various stations, but also when compared 

 with the regular course of the isobars. On drawing up the courses of the wind how- 

 over {vide fig. 59) we find that the directions taken by the wind form part of a high- 

 ly regular, and strongly oscillating wind system. Very much the same thing doubt- 

 less takes place in the sea. The observations of velocity, which at a first glance appear 

 most irregular, might possibly turn out to be in actual fact, very regular and uniform, 

 If only w^e could discern the oscillating system to which they belong. 



Naturally, it will be out of the question to procure a network .system of stations 

 at sea as close as that for simultaneous observations on land. This would require too 

 great a number of vessels. It would seem reasonable to ask, however, if the facility 

 with which it is possible to ascertain conditions in a vertical direction in the sea, 

 might not compensate for the greater facilities on land in a horizontal direction. 

 Where the surface water, owing to the oscillating movement, is massed together, the 

 surface layer must be thicker than where it flows apart. It should be an easy matter 

 to construct an instrument for indicating, for instance, the depth of an isotherm 

 situated not too far down, while the vessel lay motionless at a hydrographical station. 

 If the depth be constant during the time the measurements are being made, then the 

 sea is not in oscillation, and the hydrographical observations may be taken as repre- 

 sentative of the place and season ; if, however, the dej)th varies, then periodical oscil- 

 lations take place, and the hydrographical conditions ascertained by measurements 



