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APPENDIX, No. I.
DESCRIPTION OF THE ROAD INDICATOR, AN INSTRUMENT
INVENTED BY MR. MACNEILL FOR THE PURPOSE OF ASCERTAINING THE DRAUGHT OF CARRIAGES, AND THE COMPARATIVE MERIT OF ROADS; WITH TABLES OF EXPERIMENTS, ETC.
This instrument, which is described in the following pages, is capable of being applied to several very important purposes in road engineering, amongst which are the following:
First, It affords the means of ascertaining the exact power required to draw a carriage over any line of road.
Secondly, It can be applied to compare one line of road with another, so as to determine which of them is the best, and the exact amount of the difference, as regards horse power, both for slow and fast coaches.
Thirdly, The comparative value of different road surfaces may be determined with great exactness.
Fourthly, It affords the means of keeping a registry, in a most accurate manner, from year to year, of the state of a road, showing its improvement or deterioration, and the exact parts in which such improvement or deterioration have taken place.
* This paper has been furnished by Mr. Macneill.
PRACTICAL EXAMPLES EXPLANATORY OF THE FORE
GOING STATEMENT. 1st, Let it be required to determine the expense of working a four-horse coach over the line of road from
to at a velocity of ten miles an hour. Suppose the instrument has been run over the road, and that it has been found that the average power required to draw a four-horse coach over the whole line amounts to 350 lbs., and the distance equal to twelve miles. Let the average power which a horse should exert for eight miles a day, with a velocity of ten miles per hour, be assumed equal to 60 lbs., then 60 x 8=480 lbs., raised one mile in the day; and taking the daily expense horse equal to six shillings, we have 480 lbs. : 6s. :: 1 lb. : :15, the expense of horse power, exerting a force of one pound over one mile. Thence, 350 x •15 12 miles=630 pence, or 21. 12s. 6d., the expense of horse power required to work a four-horse coach per day over such a road.
2dly, Suppose it be required to determine whether it is more expensive to work a coach over the stage from A to B, or over the stage from C to D, both stages being exactly ten miles, and horse keep the same in both districts. Let the instrument be run over both stages, and suppose the average power thus determined to be 280 lbs. on the stage from A to B, and 320 lbs. on the stage from C to D, the difference is 320-280=40 lbs.; and this difference will amount to 40 x 10 x·15=5 shillings, in horse power, in favour of the stage from A to B.
Again: suppose the stage from A to B, which is ten miles in length, to be compared with the stage from E to F, which is only eight miles in length, but more hilly, or having a worse surface. Let the instrument be run
over each stage as before, and suppose the average power from E to F to be found equal to 500 lbs., whilst the average power over the stage from A to B is only 320 lbs., as this stage is ten miles in length, the expense of working over it will be 320 x 10 x 15=576 pence; and the expense over the stage from E to F will be 500 x 8 x·15=600 pence; from which it will be seen that less expense will be required to draw the carriage from A to B than from E to F, although the distance from E to F is two miles shorter than from A to B; and that the difference of expense will be 600-576=24 pence, or two shillings per day for a four-horse coach.
3dly, Suppose it be required to determine the best surface on different parts of a road, which has been constructed on different principles or repaired with different descriptions of road materials. Let the instrument be run over each portion of the road, and the average power noted — also the rates of inclination, as shown by the instrument, or a spirit level — then reduce the average draught over each rate of acclivity to what it would be if it was horizontal; the comparison of the corrected draughts will show the friction arising from the surface in each case. Thus, suppose the average draught over a portion of the road, which has been repaired with gravel, and which rises 1 in 20, to be 250 lbs. The correction for 1 in 20 is 39.2 lbs. The friction of the surface and axles is therefore 250-39.2, or 210-8 lbs. (See 7th Report of Parliamentary Commissioners of the Holyhead and Liverpool Roads, published by order of the House of Commons, January, 1830.)
In the same way, suppose the draught over another portion of the road which rises 1 in 10, but which has been repaired with granite, is found to be 260 lbs. The