"The divisions on the dial-plate of the dynamoineter decrease from zero upwards, in order to compensate fbr the increased force which the spring exerts in proportion as it is wound up; hi consequence of this, the index does not pass over equal spaces, when equal forces are applied in different states of tension of the spring ; the piston, therefore, will not pass through equal spaces in the cylinder, and the vibrations would consequently be greater in the higher numbers, because, the velocity of thepiston being leas, its resistance through the fluid will be less, at the same time the power oppaed to it is greater. To obviate this, and make the index equally steady on all parts of the dial, a narrow slip of brass, formed into an inclined plane, is soldered to the inside of the cylinder, parallel to its axis, the largest part being at that end of the cylinder towards which the piston rises, when the index moves towards the greater power. The notch, which was before mentioned, as cut in the side of the piston, exactly corresponds in size with the largest part of this inclined plane, so that when the piston is at the upper end of the cylinder, the notch is completely filled up by the inclined plane; on the contrary, when the piston is at the lower end of the cylinder, the notch is open ; by this contrivance, the aperture through which the fluid is obliged to pass, as the piston moves from the lower end of the i cylinder to the higher, s gradually contracted, and of course the resistance of the piston through the fluid gradually increases, and compensates the increased power of the spring, rendering the vibrations nearly uniform from the lowest to the highest power.
" To preserve the instrument from injury, it is embedded in a solid block of elm, which can be screwed or clamped to any carriage ; the swingletree is hooked .into the eye of the draught-bar ; the shafts or pole of the carriage may remain in their ordinary position, but care must be taken that no part of the moving power is communicated to the carriage, except through the agency of the instru ment. The draught of a carriage is ascertained as follows :---One assistant walks along by the side of the carriage, and observes the weight, or force, shown by the index on the dial ; at every step he calls out the numbers, which another assistant writes down in a book ; these numbers are then added together ; the sum divided by the number of observations, will give the mean power, or draught, required to draw the carriage over that portion of the road." By a very ingenious contrivance, Mr. Macneill also practically ascertained the cor rection necessary for the differeti rates of inclination. Thus, the instrument affixed to a common waggon, which was drawn by two horses over the pave. ment in Piccadilly, from the Duke of Devonshire s house to Dover-street, the sum of observations is 670 lbs., which divided by the number of observations 19, gives the mean force, 48i lbs. The street rising 1 in 156, it is necessary te apply the appropriate correction, which by the table is 15 lbs., hence, 413i — 16 = 33 lbs., horizontal draught. In a note appended to the paper, it is mentioned that the instrument has been further improved by Mr. Macneill, it is now mounted in a light phaeton, and besides marking the draught at every ten or twenty yards, it points out the distance run, and the rates of acclivity or declivity on every part of the road. The general results, as
ascertained by this useilil instrument, are stated to be as follows :— The wear and tear of roads was a point to which the attention of the Select Committee of the House of Commons upon steam carriages, in 1831, was directed. The evidence given by Messrs. Telford, Macneill, Mac Adam, Gur ney, Farey, and Alexander Gordon, proves, beyond doubt, that the destruction of a road is caused by the feet of the horses travelling, in a much greater pro portion than by the wheels of carriages. Mr. Macneill estimates the injury done by the wheels of fast coaches, to the injury done by the horses which draw them, as one to three in round numbers, or as follows :— And Mr. Gordon calculated that a set of tires would run 3000 miles in good weather ; or, on the average, 2700 miles ; but that a set of horses' shoes would travel only 200 miles. These are facts which press on our consideration the necessity of withdrawing a power so injurious to our roads as horses are described to be, and the substitution of a means of locomotion less hurtful. This desideratum will be found in the establishment of steam carriages on turn pike roads, having granite tramways • and we hope the period is not distant when this important change will be effected.
The next branch of our subject is the various patents which have been granted for improvements in the construction of roads; under which head we shall make a few remarks on pavements. The most eminent engineers have expressed themselves decidedly in favour of paving, as more durable, and, in the end, cheaper than any other mode of formation. The immense traffic in the streets of London, and other large cities, and the inconveniences resulting from a frequent derangement of the pavement, have long rendered the establish ment of a firm, durable, and smooth city road, a great desideratum. The alternate dust and mud on broken stone roads have proved them unfit for crowded thoroughfares. They have been tried, but failed. Stone paving of various kinds, and even cast-iron plates, in the form of a causeway, have been suggested. Of the two kinds of stone pavement with which London, Dublin, and Edinburgh, is paved, the one is termed ruble causeway, the other aisler causeway. In the former the stones are very slightly hammer-dreseed ; in the latter they vary from 5 to 7 inches in thickness, from 8 to 12 in length, and about a foot in depth. The Commercial Road of London is a fine specimen of the aisler causeway. It leads from Whitechapel to the extensive establishments of the East and West India Docks, at Blackwell and Poplar. It is 2 miles long, and 70 feet wide. The footpaths are laid with Yorkshire flags, and the stoneway of granite. The tramway is composed of large stone blocks, 18 inches wide by 12 mches deep, and from 21 to 10 feet long. They are laid in rows four feet apart, on a hard gravel bottom, or a concrete foundation, and have their ends closely and firmly jointed to each other, so as to prevent movement, either lateral or longitudinal. On this tramway a waggon weighing, with its load; 10 tons, was drawn by one horse from the West India Docks, a distance of 2 miles, rising 1 in 274, at the rate of nearly 4 miles per hour. The works were executed under the direction of Mr. James Walker, the engineer, by whom the plans were furnished, and whose report to the trustees of the road contains much useful information.