280 
*¢ It appears in general that, among the Alps, the formation 
of such currents depends on the setting and rising of the sun. 
In proportion to the clearness of the sky at any locality, this 
law will be found to hold with more or less exactness, and, 
therefore, although established for the interior of the Conti- 
nent, we cannot, ad priori, expect to find it realized in the 
analogous phenomena which may be observed in this country. 
Considerable changes of temperature may take place in Ire- 
land, from the variable brightness of the sky, and the influ- 
ence of oceanic currents. We should, therefore, be prepared 
to find conditions arising, which would cause ascending or 
descending currents at periods of the day that would not 
be at all anticipated by the observations made among the 
Alps. 
‘*T proceed to apply these observations to the phenomenon 
noticed by Mr. Barton. If we conceive an inland sheet of | 
water, bounded by hills on at least one side, the alternations 
in temperature between the air at the top and at the foot of 
these hills will produce such currents as I have indicated. If 
the hills be very steep towards the lake, the upper masses of 
air will sometimes even flow over the escarpment in a kind of 
cascade, and falling downwardsin a direction more or lessinclined 
to the vertical, will ultimately strike the waters of the lake in 
the neighbourhood of the hills. The more the direction of 
the descending current approaches the vertical, the greater 
will be the pressure on the lake, and therefore its effect in 
producing waves. Whenever a breeze tends to propagate 
waves on the surface of a sheet of water, its effective action is 
due chiefly to the vertical component of the force with which 
it strikes the water. Very feeble currents descending verti- 
cally might thus produce greater disturbance on the surface of 
a lake than strong currents skimming it horizontally. Such 
vertical currents descending only close to the hills at one side 
of the lake would not be felt at the other. Thus, during per- 
fectly calm weather, a heavy swell might be observed at one 
