37 



the motion of duids, not previously noticed, and susceptible of prac- 

 tical application, namely, the circumstances of the motion of an inde- 

 finitely extended non-elastic fluid, where agitated by a solid ellipsoi- 

 dal body moving parallel to itself, according to any given law ; always 

 supposing the body's excursions very small compared with its dimen- . 

 sions. The question here stated is considered by the author to ad- 

 mit of an easy general solution. As the principal object of his paper 

 is to determine the alteration produced in the motion of a pendulum 

 by the action of the surrounding medium, he insists more particu- 

 larly on the case where the ellipsoid moves in a right line parallel to 

 one of its axes ; and endeavours to prove that, in order to obtain the 

 correct time of a pendulum's vibration, it mil not be sufficient merely 

 to aUow for the loss of weight caused by the fluid medium, but that 

 it will likewise be requisite to conceive the density of the body aug- 

 mented by a quantity proportional to the density of this fluid. Ho 

 determines the value of the quantity last mentioned, when the body 

 of the pendulum is an oblate spheroid, vibrating in its equatorial 

 plane; and finds, that when the spheroid becomes a sphere, the 

 quantity is precisely equal to half the density of the surrounding 

 medium. Hence in the last case, the true time of the pendulum's 

 vibration is obtained, if it be supposed to move in vacuo, and its 

 mass be simply conceived to be augmented by half that of an equal 

 volume of the fluid, whUe the moving force with which it is actuated 

 is diminished by the whole weight of the same volume of fluid. 



8. Observations on the Fossil Fishes lately found in Orkney. 

 By Dr TraiU. 



The geologist has been for some time acquainted with the occur- 

 rence of Fossil Fishes of Caithness, and they have been more lately 

 found also in Orkney, especially near Skaill, the seat of W. G. 

 Watt, Esq. in Pomona. 



The author describes these fishes as imbedded in a dark-coloured 

 flag, which lies beneath three feet of soil and loose stone, and eleven 

 feet of solid beds of similar flag, but destitute of organic remains. 

 The fishes are contained in two strata, measuring together about two 

 feet in thickness. Tlie upper stratum contains only fishes belonging 

 to the CaHilaginei, and seemingly the genus Raia ; the lower con- 

 tains numerous fishes that belong to the orders Thoracici and Abdo- 

 minales, most of them with distinct scales. Almost aU of them lie 

 on their bellies or sides, none on their backs, and their attitudes 

 generally bespeak the energy of then: final struggles. The fishes of 

 these two contiguous strata are never intermixed. The strata dip 



