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Proceedings of the Royal Society of Edinburgh. [Sess. 
because, if there were any pressure, the moment the steam escaped from 
the confinement of the tube it would at once expand to a much greater 
diameter; but the jet shows no signs of expanding — it flows straight out, 
only slowly increasing in diameter as its motion is retarded by its being 
mixed up by eddies with the air. In the jet, therefore, the whole 
of the potential energy of pressure of the steam is converted into energy 
of motion. 
It is well known that this energy of motion in the jet can be 
reconverted into potential energy of pressure. If another pipe of the 
same diameter as the one from which the steam is issuing be placed 
directly opposite it, the jet will raise the pressure in that pipe to the 
pressure of the boiler from which the steam is issuing. Practically 
we may not quite accomplish this, owing to loss of energy due to 
friction. The points to be kept in mind from this illustration are that 
gases moving from a higher to a lower pressure lose potential energy and 
gain velocity, and that gases having velocity gain pressure while losing 
their velocity. 
Let us now apply these principles to the circulation in our atmosphere, 
and see how far they help to explain the difference in level of the strato- 
sphere over cyclones and anticyclones. In the anticyclone the pressure is 
high and the motion very slight, and the energy is all potential. On the 
other hand, in the cyclone the pressure is lower but the motion is very much 
greater. Some of the potential energy which was in the anticyclone is 
converted into energy of motion in the cyclone : what is lost in pressure is 
gained in velocity. This distribution of energy is retained by the cyclone, 
the air circling inwards and upwards to great heights, up as far as the 
strato-cumulus clouds. Above that level the circulation seems to change 
and begin to flow outwards towards the high-pressure area. The air in its 
movement towards the anticyclone loses its velocity, and in so doing regains 
pressure. If that represents what takes place, we can easily see why the 
isothermal layer is lower than the average over cyclones, because the air 
there has great velocity but has not regained its pressure. While its 
centrifugal force confers on it a quasi-horizontal pressure which enables it 
to act against the side pressure, it has no corresponding vertical pressure ; 
hence the descent of the stratosphere in that area. On the other hand, the 
low-pressure air with high velocity flowing out of the upper part of the 
cyclone towards the anticyclone loses its velocity and regains sufficient 
pressure to enable it to enter the upper part of the anticyclone, where it 
gives rise to an increase of pressure which tends to push the isothermal 
layer higher than the mean. It would thus appear that the change of 
