Screw.—Here the advantage gained is as much as the circumference of a circle described by the handle of the winch, exceeds the interval or distance between the spirals of the screw.
There are few compound engines, but what, on account of the friction of parts against one another, will require a third part more power to work them when loaded, than what is required to constitute a balance between the power and the weight.
Ferguson’s Nat. Philosophy
MILE.—Comparison of the different miles, in geometric paces, each of which is equal to 5 feet French royal, 5.6719 feet Rhinland, or 6.1012 English feet.
| The mile of | Sweden | = | 5761 | geometric paces. |
| ” ” ” | Switzerland | 4512 | ||
| ” ” ” | Denmark | 4071 | ||
| Common, of | Germany | 4000 | ||
| ”” | Holland | 3158 | ||
| League of | France | 2400 | ||
| ” ” | Spain | 2286 | ||
| ” ” | Scotland | 1500 | ||
| Mile of | Italy | 1000 | ||
| ” ” | England | 868 | ||
| Werste of | Russia | 575 |
MINE.—The excavation formed by the blowing up of a mine is found by experiment to be nearly a paraboloid. It was formerly supposed that the diameter of the entonnoir, or excavation, was always equal to only double the line of least resistance; but experiments have proved, that the diameter of the excavation may be increased to six times the line of least resistance; and that the diameter of the globe of compression may be increased to eight times that line; this is called the maximum of a mine, or the greatest effect that can be produced by a globe of compression. In any mine intended to produce an effect within this extent, the effects will be nearly as the charges.
The globes are to each other as the cubes of their radii. Their radii are the hypothenuses of rightangled triangles, of which the line of least resistance, and the semi-diameter of the excavation, are the other two sides. Therefore, to find the charge to produce any required diameter of the excavation, the following will be the rule, the radius being found as above:
As the cube of the radius of the globe of compression in the following table, (having the same line of least resistance as the required globe,)
Is to the cube of the radius of the required globe;
So is the charge corresponding in the following table,