Mechanical Science. 381 



they are supposed to be freely suspended, they are made to 

 approach until within a determinate distance of each other ; if 

 this distance be greater than the double thickness of the liquid 

 layer adhering to each, the two envelopes will be merely placed 

 opposite to each other, but will not mutually penetrate ; and then 

 the two moistened surfaces obeying the gravity which they retain 

 in the fluid will, like insulated pendulums, re-assume the vertical 

 position from which they have been moved. Now this will take 

 place in a certain interval of time depending on the length of the 

 thread or wire by which the surfaces are suspended, and the re- 

 sistance which the fluid opposes to their motion. It is also evi- 

 dent that, on abstracting this resistance, which can always be done 

 when the motion of the pendulum is extremely slow, the duration 

 of the oscillations will be the same, whatever the interval through 

 which they have been moved from the perpendicular. 



But if, assuming another case, the two solid surfaces immerged 

 are brought so near to each other that the fluid layers which 

 adhere to them mutually penetrate, they will then mutually attract 

 each other ; the action of their weight in the liquid will be in 

 part counteracted by this attraction ; and when they are left to 

 themselves, the time they will require to attain the original per- 

 pendicular situation, i.e., the duration of the first half oscillation, 

 will be the greater as their mutual attraction increases, or as they 

 are placed at a smaller distance from each other at the commence- 

 ment of the oscillation. 



The effort of gravity on the moistened surfaces may be rendered 

 feeble to any degree, either by giving to the surfaces in one way 

 or another, a specific gravity, but little different from that of the 

 liquid in which they are immersed ; or in diminishing the angle 

 formed by the suspension wires when they are removed from the 

 perpendicular, and made to approach each other. Then by inter- 

 posing and fixing between the surfaces a metallic wire of a certain 

 diameter, and by means of pressure, bringing the surfaces in con- 

 tact with the opposite sides of the wire, it is evident that the 

 diameter of the latter becomes a measure of the interval between 

 the surfaces. If, then, the surfaces be left to the contrary actions 

 of their mutual attraction, and their gravity in the liquid esti- 

 mated parallel to that attraction, the duration of their first oscil- 

 lation will necessarily be a certain function of the difference of 

 these contrary forces. If then this duration be observed whilst 

 the forces are varied, i. e., in varying either together or separately 

 the diameter of the interposed metallic wire, and the amplitude of 

 the movement of the surfaces, the observations will indicate the 

 mutual variations of the distance of the two surfaces at the com- 

 mencement of the oscillation, the amplitude of the oscillation 

 itself, and also its duration. 



M. Girard then proceeds in his memoir to describe an ap- 

 paratus constructed with great care according to these views. 

 Vol. XX. 2 D 



