SIR G. B. AIRY ON THE TIDES AT MALTA. 
137 
longest period occurs more frequently at Malta than in Switzerland. The origin 
assigned by Dr. Forel is, I think, most certain ; that they are waves originally caused 
by winds ; but that they are reflected from one side and another of the limited sea, 
and thus become stationary waves. The waves forming the seiches of Malta are 
reflected, I suppose, from the shores of Sicily and Africa. 
On examining the chart of those seas, it appears that a large bank, covered by 
comparatively shallow water, projects from the African coast, bounded roughly by a 
li n e nearly from Cape Bon to the island Linosa ; and that between that bank and a 
narrower bank on the Sicilian side there is a broad sea- channel of approximately 
uniform width and of extremely deep water. I imagine that the reflection of 
undulations takes place principally, not at the shores of the land, but at the edges 
of the banks bounding the deep water. I have not yet ventured on a numerical 
calculation, but in rough estimate it appears to me that the breadth and depth of this 
sea would hydrodynamically explain the return of waves at periods of 21 m . Such 
waves, once created, would be propagated to regions of the sea somewhat beyond 
those in which they are formed. 
For closing this account, I think that nothing more is required than to append 
a table descriptive of the seiches on the days in which they are large enough to 
attract attention. I have thought it sufficient to state the number of undulations 
occurring in a group between two limits of time, thus giving the average length in 
time of an undulation, and to state the greatest elevation or depression measured 
from the luni-solar tidal curve which occurs in the group. The limits of the groups 
are quite arbitrary, having been determined in most instances for convenience by the 
length of the sheets of paper on which the measures were transcribed. 
MDCCCLXXVIII. 
