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If we suppose the power which a horse usually exerts in mail and fast stage coaches to be equivalent to a constant pull of 37 lbs.i over ten miles in a day, with a velocity of ten miles an hour, the effect will be equal to 651,200 lbs.; for 1760 x 10 = 17,600 yards in 10 miles, and this sum multiplied by 37 lbs. equals 651,200 lbs. drawn over one yard in the day; which number may be taken as a standard
* This is the power assigned by Mr. Trcdgold, in his work on Railways, as that which a horse should exert working with a velocity of ten miles an hour, which, though sufficiently correct, as in the present instance, for a measure of comparison, should not be considered as a fixed standard of the power of a horse working at the velocity of ten miles an hour, as the formula which Mr. Tredgold has used appears to be founded on a limited number of experiments.
for horse power in comparing one line of road with another.
If, on this principle, we know the average draught over any line of road, and the length of that road in yards, we at once know the horse power to which it is equivalent, and, consequently, can compare it with any other line.
Thus, the average draught on the old road between Barnet and South Mims, multiplied by the length of that road in yards, is equal to 165,320; by the new line, the average draught multiplied by its length in yards is 139,028*; the difference between this and the old road is 26, 292, which, divided by 651,200, the power of one horse, gives i04 part of a horse power; this multiplied by 500, the number of horses travelling over the road each day, and supposing each horse to be worth five shillings per day, the saving by making the new road will amount to 5/. per day, or to 1,800/. annually.
The result derived from this calculation depends not on any theory or abstruse calculation; it is a matter of fact, and cannot be disputed; and is, perhaps, one of the most useful practical applications of the Road Instrument; for without it the most refined and difficult application of algebra to the plans and sections of the above roads would only give an approximate value, as the state of the surface could not be brought into the calculation, except by guess, and this would be little better than judging by the sections, as heretofore practised, without any decided or fixed principle.
* See the annexed table of the actual draught on every twenty yards of this road.