210 Proceedings of the Boyal Society 
the direction of the less copious source of supply of the lowest 
gradients, and of the widest isobars, accompanied of course by high 
pressure and by steep gradients on the south segment, which there 
necessarily aids the velocity of the west winds, and in the direction 
of progress. Owing to the complexity and difficulty of the mode 
in which spiral inflow takes place, and also of the direction in which 
it is altered in the different segments, absolutely dependable obser- 
vations are not to be found. For facility of explanation, it is here 
taken up in two points of view. First, in the mode of direct inflow 
which only occasionally takes place ; and, secondly, also in the very 
important mode in which it circulates round the low centre, but 
which, as exhibited on the surface, takes place in a very different 
manner on the different segments. It is only by circulation that 
the structural form of an aerial cavity can possibly be maintained. 
This circulation, which is in a direction against that of the hands 
of a watch, is for facility here supposed to take place at the same 
rate of speed as that of progress, and in an easterly direction. On 
the north segment, east winds, which here circulate round the low 
centre and in a direction opposite to that of progress, are necessarily 
calmed, though only with reference to the surface. On the south 
segment, where west winds both circulate and blow in the direction 
of progress and with a consequent amount of velocity, they must 
necessarily there be accompanied by high pressure, while east winds 
only require low pressure, and which must also be found in front. 
The velocity of east and west winds are here supposed to circulate 
at the same rate of speed round the low centre ; with reference to 
the usual surface observations, they are not here comparable. East 
winds may thus be regarded as being those of space. It is owing to 
this, as shown, that there is to be found the remarkable difference 
in the alteration of the position of the north and south segments in 
the form of a circular atmospheric wave. This is fully illustrated 
in the diagrams of the paper, and is here shown how the calm north 
segment of east winds can only alter its position, and by so doing 
exhibit the necessary progressive movement. 
In a letter received from Ballot, he mentions his great difficulty 
in ascertaining the comparative velocity of east winds with reference 
to their gradients as compared with those of other winds. As 
above described, how can the effect of the gradients of the different 
