Chap. XIII.

CARRIAGES FOR TRANSPORTING WEIGHTS.

1017

The erection of a vase or statue onits pedestal is generally performedbv three movements, one vertical, toraise the object to the requiredheight, another to move it horizon-tally, and the third to fix it in itsplace.

The most simple method is by theuse of tackle attached to the top of amast, or two poles lashed togetherat a height sufficient to enable theobject to be lodged where required :such a shear or support is representedin the cut; its position on the groundbeing established by pulling it up-right, the base is hung directly overthe pedestal; after which it maybe lowered by means of pulleys atpleasure. The term shear is ap-plied to the arrangements made inour dockyards by which the mastsare hoisted into ships: one atWoolwich is an excellent example ofthis kind of tackle. The polyspastonof the Romans was no doubt asimilar machine in every parti-cular.

Tractorial machines, as they aretermed by Vitruvius , consist of twomovements, different and dissimilarfrom each other, but so agreeingthat the operation is perfected byboth; one is rectilinear, and the othercircular, but neither alone can bemade effectual for raising weights;when stones were raised for buildingpurposes, the forfices or forcipeswere sometimes used: and Piranesiin his u Antichita Romana” assertsthat he found, in some of the an-cient buildings, stones with holes cut

in their upper faces, in such a form Fig. 1539. raising a vase.

as to suit the dovetail shape of the modem Lewis or Louis; there can be no doubt butthat this instrument was used by the Romans.

CHAP. XIII.

CARRIAGES FOR TRANSPORTING WEIGHTS, AND BOATS, ETC.

Carriages for transporting Weights and Boats , §*c. — The daily work of a horse has beenestimated as equal to that of five or six men; its immediate force may be somewhat greater,but it cannot work profitably for more than 8 hours a day when pulling with a force of200 pounds, or 6 hours when exerting that of 260 pounds, and walking miles per hour:when drawing up a hill the horse loses much of his power, as he does when the carriage towhich he is attached passes over a yielding or bad road. Whenever any large quantity ofearth or other material is to be carted away, it is found profitable to lay down a provisionaltram or railroad ; but where this is not practicable, the carts or waggons should be welladapted to the use for which they were intended. The waggons generally employed onour railways whilst the works are in progress are without springs, and are now ordinarilydrawn by locomotives or fixed engines, at the rate of 8 or 10 miles an hour.

The dynamical effect of a horse employed for raising earth-work up an inclined plane,the base of which was 60 feet, and the vertical height 40, was found only equal to a resis-tance of 410 pounds when travelling through a space of 72 feet in 1 minute, which is equalto 14,760 pounds raised 1 foot high per horse power per minute.