128 Proceedings of the Royal Society of Edinburgh. [Sess. 
“ Clacton : E. at 18h. and 21h. 
Nottingham : N.E. at 18h. and 21h. 
Dungeness : N.E. at 21h. 
Dover : S.E. at 18h. 
Holyhead : S. at 18h. 
Pembroke : E. at 18h. 
The term ‘ upper cloud ’ is rather vague, and there is no reason to suppose 
that the clouds referred to on this date were at any considerable height. 
The motion is that of the lower cloud sheet. 
“ The possibility of deducing the pressure distribution and wind currents 
at two or three kilometres above ground from the surface pressures and 
temperatures shown on the 18h. working chart has been considered. In 
view, however, of the high probability of inversions of temperature over 
the cold continental area, there is no satisfactory way of estimating the 
average temperature gradient. In all probability the Easterly current was 
of limited extent, but there seems to be no way of judging the height of its 
upper boundary.” 
From the observations of my correspondents, it would seem that over 
most of the inner sound-area there was little or no wind. The only record 
which I have received is one of north-east at Croydon. In the outer sound- 
area the same stillness characterised the neighbourhoods of Grantham, 
Hunstanton, and Wells. In other parts of Norfolk, and near Beccles in 
Suffolk, however, the wind was fairly strong, and as a rule from between 
east and north-east. 
I have given these details though, in the absence of observations at a 
considerable altitude, they appear to throw but little light on the forms and 
directions of the sound-areas. While they may account for the limitation 
of the inner area towards the north-east, they furnish no explanation of the 
limitation of that area towards the south or of the existence of the outer 
sound-area. In considering the paths followed by the sound-waves and 
air- waves, we must rely on observations made on the surface of the ground. 
Paths followed by the Sound-Waves and Air-Waves . — The existence 
of the silent zone implies that the sound-waves took an upward course 
while travelling across it ; and the prevalence of a contrary wind, the 
velocity of which increased with the height above the ground, would be 
sufficient to cause the waves to leave the surface of the ground. That 
they began to rise in their progress outwards over the inner sound-area is 
shown by the rapid decline in the intensity of the sound within that 
area. The intensity of the inaudible air-waves declined less rapidly 
