as Associates. No. 800.--"On the Obstructions to Navigation in Tidal Rivers." By John Thornhill IIarrison, M. Inst. C.E.THIS subject naturally divides itself into the two considerations of the powers of the solid materials which .form the obstructions to navigation; and the action of water, which is the chief agent in giving motion to those materials. I t is proposed to consider first, in a general manner, the circumstauces which affect the deposition of the solid material and the action of the water,-and secondly, to apply these circumstances, in explanation of the formation of obstructions existing in the bed and at the entrance of tidal rivers.1st.-The obstructions to navigation are composed of the solid materials of the earth, which being deposited in various localities, form banks of gravel, sand, or mud; thus lessening the depth of water and becoming detrimental to navigation. Now these materials, as usually met with, composing the crust of the earth, are in a compact form, the particles being held together by their cohesion. When the cohesion of the mass is destroyed, and the materials are broken up into small fragments, the friction of one part against another, prevents the whole from assuming a level surface and being drawn by the force of .gravity, as nearly as it can be, to the centre of the earth, like water, which is, in a great measure, devoid of cohesion and friction. The limiting angle, at which various kinds of solid materials stand erect, or at given angles, is thus soon attained, and the application of force is necessary to move them from their position.Water soaking into materials frequeutly lessens the cohesion of the particles; it also diminishes friction, so as to cause the motion of solid bodies, by which they are brought within range of the action of water in motion. Land-slips, falls of cliffs, &C., are the frequent effects of these causes. I n cases where the cohesion is entirely destroyed, the material, when mixed with water, becomes semi-fluid, and will only rest in a horizontal position. A very slight movement of the water .is sufficient to disturb it from its state of rest, whereas it requires greater velocity to communicate motion to