RESISTANCE is a power by which motion, or a tendency to motion, in any body is im peded or prevented. When a weight or pres sure acts upon a beam or bar in any direction, the tenacity by which the particles of such material oppose that action constitutes a re sistance of one kind. [MATERIALS, STRENGTH GE.j Again, when a body is made to move on another, the inequalities of the surfaces of both create a resistance of a different kind. [F11=1011.1 When a body moves in a fluid, the inertia of the find particles displaced by it produces a third kind of resistance. It is computed that a 24-pounder ball experiences a resistance equal to 800 lbs. when its velocity is equal to 2000 feet per second. Like effects take place in the movement of boats or ships ; when the velocity is great, the water accumulates in front, and flowing off from thence obliquely, it carries away some from the sides, and, causing the surface of that which is near the stern to be rather lower than the general level, it there produces a diminution of pressure, while there is an excess in front on account of the accumu the experiments of Mr. Telford, the
following values of the resistances experienced by loaded carriages on level roads have been determined. On a good pavement the re sistance is of the weight of the carriage and load ; on a broken surface of old flint, ; on gravel, A.; and on a well-constructed railway, from to aka. Mr. Barlow has found that, with small velocities, the force of traction on canals is less than on railways ; and when the velocity is equal to four miles per hour, the forces are equal. Beyond this velocity the advantage is in favour of the railway.
All these enquiries are of great importance in ship building, railway malting, and other engineering works, where motion is to be pro vided for.