676 
PROFESSOR JAMES THOMSON ON THE 
porizable on any occasion. It is mentioned in the printed abstract briefly in the 
following: words :—“ If a shallow circular vessel with flat bottom, be filled to a moderate 
depth with water, and if a few small objects, very little heavier than water, and 
suitable for indicating to the eye the motions of the water in the bottom, be put in, 
and if the water be set to revolve by being stirred round, then, on the process of 
stirring being terminated, and the water being left to itself, the small particles in the 
bottom will be seen to collect in the centre. They are evidently carried there by a 
current determined towards the centre along the bottom in consequence of the centri¬ 
fugal force of the lowest stratum of the water being diminished in reference to the 
strata above, through a diminution of velocity of rotation in the lowest stratum by 
friction on the bottom. The particles being heavier than the water, must, in respect 
of their density, have more centrifugal force than the water immediately in contact 
with them ; and must, therefore, in this respect have a tendency to fly outwards from 
the centre, but the flow of water towards the centre overcomes this tendencv and 
carries them inwards ; and thus is the flow of water towards the centre in the stratum 
in contact with the bottom palpably manifested.” 
The general hydraulic principle intended thus to be illustrated by the exhibition 
of an easily conducted simple case of it is, that if water were lying on a revolving 
flat-bottomed circular plate or tray, and were revolving at each part quicker than the 
tray immediately below that part, a flow would institute itself in the bottom layer 
towards the centre, and that this would occur alike for different speeds of revolution 
of the tray, and would still take place, likewise, in the case of the speed of revolution of 
the tray being abated to zero. The case of the non-rotative tray was taken for illustra¬ 
tion of the more general proposition simply because of the facility which that particular 
case presents for being brought into visible manifestation, so as to form to an intel¬ 
ligent mind a help to the imagination in considering the action of the great cap of 
air lying on the middle and higher latitudes, and revolving prevalently at each part 
quicker than the Earth below that part does. I offer these explanatory remarks here 
because in a paper by Mr. Fereel, to be told of a little further on, my illustration by 
means of the non-revolving tray has been made a point of adverse criticism as to both 
the nature and the value of the theory I had offered. 
Now, before passing quite away from the subject of the original framing of my own 
theory ; I feel it right to make special reference to two considerations which were put 
forward by Mr. Fekrel in his paper of October, 1856. 
Firstly .—Ideas were put forward in that paper by Mr. Fere el to the effect, that 
the low barometric pressure found observationally to exist in polar regions and other 
high latitudes, is due to the centrifugal force or tendency of the air of the surrounding 
middle latitudes revolving from west to east quicker than does the earth below : but 
his views on the matter being unknown to Mr. Murphy and to myself, did not happen 
to influence my considerations. 
And secondly, Mr. Feerel in that paper adduced in connection with other suppo- 
