612 
Proceedings of the Royal Society 
Monday, 5th July 1875. 
Sir EOBEET CHEISTISON, Bart., Hon. Vice-President, 
in the Chair. 
The following Communications were read 
1. The Theory of the Causes by which Storms Progress in 
an Easterly Direction over the British Isles, and why the 
Barometer does not always indicate real vertical pressure. 
By Eobert Tennent, Esq. 
Upwards of three years ago the author laid a paper before two 
members of the Scottish Meteorological Society. The question 
taken up was, why horizontal movement takes off vertical pressure ; 
and the conclusion arrived at was, that every such horizontal cur- 
rent, owing to its passage over a resisting surface, and by means of 
rapid upper currents, caused removal of air and lifting, and thereby 
diminished pressure. It was inferred that the barometer which re- 
presented this was consequently an “effect” and not a cause of wind. 
The present remarks will be confined mainly to the mechanical 
effects of motion and friction, — the important questions of tempera- 
ture, vapour, rotation, external high and low pressure, &c., not being 
now considered. 
Friction . — This forms a very important element. To it is due 
the retardation of the surface currents, while the upper currents 
move more rapidly, being comparatively free and unimpeded. 
Surface retardation is increased by pressure, which amounts to 8J 
tons on every square yard, but this gradually diminishes upwards. 
Tyndal, by experiment, estimated the mobility of the uppers on 
Mont Blanc as being twice as great as that of the surface. When 
the atmosphere is in a state of rest, its columns maybe represented 
as being vertical or upright, but when rapid uppers and retarded 
surface currents prevail, it may then be regarded as moving in 
inclined columns at an angle to the surface, and in the direction 
of the moving force. 
Supply . — The inclination of the columns will depend not only 
on surface friction, but also on the supply of air to the moving cur- 
