6. Imagine a large vessel D G H F (fig. S), which contains a fluid subjected to no forces but gravity and the attraetion of its own particles, and consequently having its surface DF horizontal ; let AB represent a rectangular plate partially plunged in the which it attracts; and supposing the surface of the fluid to remain level, let it be proposed to investi gate the force with which the attraction of the plate tends to disturb the equilibrium of the fluid.
Suppose a horizontal plane, df, to be traced in the fluid, at a depth equal to the range of the cor puscular force, then this plane will separate all the superficial strata, in which the pressure is variable, from the rest of the mass. Below the plane df, the fluid particles cohere with the same force in every part, and they are likewise attracted with equal in tensity by all the points of the plate with which they are in contact ; above the same plane the attractive force of the plate remains unchanged, but the pres sure of the fluid in the different strata is variable, gradually becoming less and less as we approach the surface. It will, therefore, be proper, first to exit mine what tendency the part of the plate below the plane df has to disturb the equilibrium ; and, second ly, to consider the effect of the plate's attraction up on the superficial film or stratum.
If the matter of the plate have the same attrac tion for the particles of the fluid that they have for one another, we may consider the plate as a body of the fluid that has congealed without any other change ; in which case, it is evident, that, below the superficial stratum, the cohesive force of the fluid particles will be equal to their adhesion to the plate, and the action of the solid matter will nowise dis turb the equilibrium of the fluid in the vessel.
If the plate be supposed to have no attraction for the fluid, a canal having one end in the surface of the fluid, and the other end on the plate, will be si milar to a canal terminating both ways in the fluid's surface. It will be in equilibrium by the mutual at tractions of the particles within it, and will exert no pressure whatever upon the plate.
If the solid matter attract the particles of the fluid, but with less intensity than they attract one another, there will be an adhesion of the fluid to the plate in proportion to the attractive force. In this case, we
may distinguish the attraction between the fluid par ticles into two parts, one of which is equal to, and in equilibrium with, the attraction of the solid mat ter; while the other part, which is over and above what balances the attraction of the solid matter, is in equilibrium by the mutual action of the particles upon another.
The solid matter acts immediately upon a thin portion of the fluid in contact with it ; that portion attracts another contiguous portion ; and, in this manner, the attraction of the plate reaches to any distance in the fluid mass. But from this it is mani fest, that the whole of a force greater than the mu tual attraction of the particles cannot be propagated to a distance. Part of it must remain confined to the sphere of immediate action. Hence, if the plate attract the particles of the fluid with greater inten sity than they attract one another, a part only of the attraction of the solid matter will balance the whole attraction of the fluid ; and the remaining part will not penetrate beyond the range of the corpuscular force, but will act only upon a thin film of the fluid in contact with the plate. In this case, therefore, . the plate's attraction produces a force which is not absorbed by the fluid. As this force compresses the thin film on which it acts upon the plate's surface, it will be attended with a lateral pressure, or an effort of the film to spread itself on all sides ; and it may at first be thought that this lateral tendency, by act ing upon the superficial stratum, will disturb the equi librium. But it will immediately occur, that the ef fort which the edge of the film adhering to the plate below the plane df, makes to raise up the superficial stratum, is counteracted by the opposite effort of the fluid situated immediately above the plane df. Thus, in every relation that can subsist between the attractive powers of the plate and the fluid, that part of the solid which is immersed below the superficial stratum, has no tendency to disturb the equilibrium of the fluid in the vessel.