Page:Popular Science Monthly Volume 18.djvu/469

Rh been recently invented, and was used for land already under tillage. He also mentions the coulter (culter). This knife, fixed in front to make the first cut ready for the share to turn the sod, is a great improvement on the primitive plows, where the plowshare has to do the whole work. In Pliny's time, though only forming part of some plows, it was evidently well known. Thus he recognizes the whole construction of the wheel-plow (Fig. 10) as figured by Caylus from an ancient gem. The ordinary modern plow used by the English farmer improves upon this rather in details of construction and material than in essential principle, though a new start in invention is taken by the self-acting plow, which no longer needs the plowman to follow at the plow-tail, and by the steam-plow, which substitutes engine-traction.

The plow, drawn by oxen or horses, and provided with wheels, has taken on itself the accessories of a wheel-carriage. But, when the plow is traced back to its earliest form of a hoe dragged by men, its nature has little in common with that of the vehicle. Though the origin of the wheel-carriage is even more totally lost in prehistoric antiquity than that of the plow, there seems nothing to object to the ordinary theoretical explanation (see Reuleaux, "Kinematics of Machinery," and others), that the first vehicle was a sledge dragged along the ground; that, when heavy masses had to be moved, rollers were put under the sledge, and that these rollers passed into wheels, forming part of the carriage itself. The steps of such a transition, with one notable exception which will be noticed, are to be actually found. The sledge was known in ancient Egypt (see the well-known painting from El Bersheh of a colossal statue being dragged by men with ropes on a sledge along a greased way, Wilkinson, "Ancient Egyptians," vol. iii). On mountain-roads, as in Switzerland, as well as on the snow in winter, the sledge remains an important practical vehicle. The use of rollers under the sledge was also familiar to the ancients (see the equally well-known Assyrian sculpture of the moving of the winged bull, in Layard's "Nineveh and Babylon," p. 110). If, now, the middle part of the trunk of a tree used as a roller were cut down to a mere axle, the two ends remaining as solid drums, and stops were fixed under the sledge to prevent the axle from running away, the result would be the rudest imaginable cart. I am not aware that this can be traced anywhere in actual existence, either in ancient or modern times; if found, it would be of much interest as vouching for this particular stage of invention of the wheel-carriage. But the stage which would be theoretically the next improvement is to be traced in practical use;