26 Dr. W. K Shaw. On the General [May 16, 



I now refer to fig. 4, showing the average distribution of surface 

 temperature for the same month, January. Again the similarity of 

 the trend of the lines to those of figs. 2 and 3 is obvious. It is of 

 course not in any way remarkable that figs. 3 and 4 should show 

 similar lines, for figure 3 shows the amount of barometric pressure to 

 be deducted from the surface pressure for a layer of air 4000 metres 

 thick, and the calculations of the deductions have been based upon the 

 surface temperatures. The diminution of pressure for a given height 

 is the same at all points of an isothermal line, assuming the vertical 

 temperature gradient to be the same at all points along the line ; but 

 that the remainders representing the pressure of the upper atmosphere 

 at 4000 metres should also be very similar in shape, is indeed remark- 

 able, especially when one considers that the gradient is in the reverse 

 direction from that of the pressure values for the lower stratum. 



It is unnecessary to call special attention to the points of similarity 

 even in detail between the shapes of the " partial " isobars for the lower 

 stratum, fig. 3, and the upper stratum, fig. 2, with reversed gradient. 

 The isolated minima in fig. 3 about the equator, corresponding to 

 isolated maxima in the same regions in fig. 2, are very striking, but one 

 point of difference may be noted. In fig. 2 the circulation indicated is- 

 approximately about the geographical pole, where, in consequence, a- 

 minimum of pressure is indicated. In fig. 3, the circulation is clearly 

 round a pole of extreme cold in north-east Siberia, where there is a, 

 maximum of pressure for the lower air. 



It thus appears that the forces represented by the average distribu- 

 tion of pressure for the month of January may be divided into two 

 parts, viz. : 



A, that due to the upper atmosphere, above 4000 metres, which 

 would, if it acted alone, correspond with a steady motion from west to 

 east along paths following closely the lines of the average isotherms ; 

 and B, that due to the lower stratum of the atmosphere which, if it 

 acted alone, would correspond with a steady motion also approximately 

 along the isotherms but from east to west. 



The actual distribution is represented by the superposition of A and 

 B. Since pressure gradient and velocity are both vector quantities,, 

 and are related to each other in simple proportionality, the super- 

 position of force distribution corresponds with the composition of the 

 velocities due to the separate distributions. Hence, in the combina- 

 tion of the two distributions of force at the surface where A is 

 predominant, i.e., in the middle latitudes on either side of the equator, 

 there is a resultant circulation from west to east; where B is pre- 

 dominant, i.e., in higher latitudes, there is a resultant circulation from 

 east to west. 



The superposition of the two systems gives a line of minimum 

 pressures, with a westerly flow of air on the equatorial side, and an 



