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SIR G. B. AIR Y OH THE TIDES AT MALTA. 
Remarking the smallness of the tide (which is little more than a finger-nail’s breadth) 
it may be considered that the tides II., III., IY. present that opposition of sign to 
VI., VII., VIII. which they ought to maintain, and that the term depending on S 
may explain some part of the remaining discordance. 
There is one mechanical consideration applying to the register of the diurnal tide 
which does not apply in the same degree to the semidiurnal tide. A single sheet at 
a time, as I understand, was placed on the revolving barrel ; and it is not easy to 
attach this without the risk of a little inclination, which would produce the appearance 
of a diurnal tide ; and there was no base-line produced by a pencil on the fixed part 
of the instrument which would give evidence on this point. No error, however, 
appears to have been produced in the mean height for the day. 
Section VI. — On the “ Seiches ” or Non-Tidal Undulations of Short Period at Malta. 
I have mentioned above the fluctuations of short period observable on the tidal 
record made by the self-registering tide gauge, and the methods of eliminating their 
effect in the treatment of the tides. During the month of the tidal discussions these 
fluctuations are small, but in some following seasons of 1871 and 1872 they became 
more important. 
Their general characteristic is ; that they are simple harmonic curves (excepting in 
the larger undulations in 1872, when their heads are sometimes notched, as by the 
intermixture of small waves originating from different causes) ; that they recur with 
great but not perfect uniformity at intervals of about 21 minutes of time; and that 
they continue for many hours at a time, sometimes for entire days ; that then- 
magnitude is very variable, sometimes small, sometimes amounting to ±6 inches (or 
producing a range of 12 inches), much exceeding that of the luni-solar tides. 
These fluctuations, occurring in a sea usually so tranquil as regards movements of 
slow character, were naturally known at Malta, and were not unfrequently attributed 
(conjecturally) to volcanic disturbances of Stromboli. I knew, however, that similar 
fluctuations were recorded by a self-registering tide-gauge at Swansea (see the 
‘ Encyclopaedia Metropolitana,’ “Tides and Waves”), and was persuaded that they 
were due to some hydrodynamic cause. 
At this time I received from Dr. Forel, of Lausanne, an account of his remarkable 
investigations on the undulations locally known as “ Seiches,” in the Lake of Geneva, 
and on similar but less conspicuous fluctuations in other lakes of Switzerland. 
(‘ Annales de Chimie et de Physique,’ 5 me serie, tom. ix., 1876 ; ‘ Archives des Sciences 
de la Bibliotheque Universelle/ August and December, 1876; May, 1877.) On 
comparing the fluctuations on the tidal sheets with the descriptions by Dr. Forel, 
and above all with his engraved diagrams, it was impossible to doubt that they 
are phenomena of the same class. Every form delineated by Dr. Forel is to be found, 
I believe, upon the Malta sheets ; but I think that the simple harmonic curve of 
