348 SCIENCE PROGRESS 



apart may be different, not because the meteorological condi- 

 tions are different but because the instruments are differently 

 affected by local obstacles to the motion of the air. I propose 

 to take the gradient velocity as the standard and will endeavour 

 to explain what I mean thereby. 



If we take an ordinary weather map upon which the dis- 

 tribution of pressure is represented by isobaric lines (see for 

 example fig. 9) we find that the observed wind at the coast 

 stations is more or less along the isobars and is strong when 

 the isobars are close together, feeble when they are far apart. 



Let me ask: What kind of motion is this wind motion? It 

 is clear that it is closely related in some way to the pressure. 

 It must be borne in mind that there are two kinds of motion, 

 both of which may be illustrated by a pendulum bob : first the 

 motion which is displayed when the bob is pulled aside and let 

 go, the other when it is projected so as to describe a circle. In 

 the one, the motion is along the line of the force and the velocity 

 increases rapidly as the bob gets towards the centre ; in the 

 other, there is no increase of velocity, only change of direction, 

 because the force is perpendicular to the direction of motion. 

 With the winds the forces are the pressure differences and very 

 little increase or decrease of velocity is produced in seconds, 

 minutes or even hours (1). Which of the two kinds of motion 

 best represents the wind ? The forces are not quite perpendi- 

 cular to the motion but nearly so ; the. persistence of the 

 velocity, with some other evidence which I will not give here, 

 is proof enough that it is the motion with the balance of force 

 by change of direction that is to be taken. The rotation which 

 has to balance the force is not pendular rotation but the rota- 

 tion of the earth ; if we can assume that the rotation of the 

 earth just balances the pressure-gradient, we can calculate the 

 relation between pressure and wind for air which moves along 

 a great circle of the earth by the formula (2) 



y = 2 cop V sin X or V = 



2 cop sin X 



We may call the velocity thus calculated from the pressure the 

 gradient velocity and the wind which has the gradient velocity 

 and is along the isobar the gradient ivind. We will use that for 

 our standard of reference. It is evidently not dependent upon 

 local exposure; for that reason, it is a better standard than 

 the local surface wind. 



