lain on the Cartesian system, but which was afterwards correctly attributed by Hawkesbee to electricity.
Wall, in 1708, observed the sparks produced from amber, and Hawkesbee noticed the sparks and snapping under various modifications.
Dufay and the Abbe Nollet were the first to draw sparks from the human body; an experiment which attracted great attention, and became a species of fashionable diversion at the time.
The discovery of the Leyden jar is attributed to Cunoeus of Leyden, in 1746, who, while handling a vessel containing water in communication with an electrical machine, was surprised at receiving a severe shock; a similar event had happened the year previous to Von Kleinst, a German prelate.
Gray in 1729 discovered that certain substances were possessed of a conductive in contradistinction to an electric power; and afterwards Nollet passed a shock through a circle of 180 men of the French guards, and along a line of men and wires 900 toises in length, while Watson
them together, by means of a metallic discharger with nonconducting handles, as shown at b, a spark is obtained.
c is a battery formed by connecting together in series all the outer and all the inner surfaces of several jars, so that the united force of the whole is concentrated in the act of discharging.
The principle of the Leyden-jar was discovered by Muschenbroeck at Leyden in 1745, hence its name.
Von Kleist in Germany made the same discovery in the same year.
Gralath in Germany, 1746, contrived the electric battery by combining a series of jars; and finally Drs. Watson and Bevis, by covering the outside of the jar with tinfoil, brought it to the complete state in which we now have it.
The Leyden-jar is a condenser, its two coatings of tinfoil performing the parts of a collecting plate and a condensing plate.
1. A small balance.
2. A level (which see).
（Carding-machine.) A drum with cards on its periphery presented at the throat o<
the compass was known before Columbus, but it was assumed to be uniform.
The first voyage of the great discoverer was nearly brought to an abrupt conclusion by the discovery of this error.
See mariner's compass.
The dip of the magnet was discovered by Robert Norman, 1576.
The magnet worn by Sir Isaac Newton in his ring weighed only 3 grains, yet was able to take up 746 grains, or nearly 250 times its own weight.
Artificial magnets were made by Dr. G. Knight in 1746.
A natural magnet is the loadstone, which possesses polarity like a magnetic needle.
A magnetized needle is a piece of steel or iron to which polarity has been imparted by contact with a loadstone, or by other means.
A manuscript of A. D. 1206, in the Royal Library of Paris, states that a black stone, called mariniere, was rubbed by sailors upon a needle, which was then placed on a straw set afloat in a basin of water, when the point would indicate the north.
See mariner's compass.
und the perimeter.
See Fig. 3202, page 1468.
A walled hole for the heavy flywheel of a train of rolls, etc.
1. A plow supported in part by a wheel or wheels as a gage of depth.
3823-3825, pages 1745, 1746.
2. A plow with a wheel in the space between the landside and mold-board, and reducing the friction of the plow by bearing the weight.
See f, Fig. 3823; see also B and C, Fig. 3825, page 1746.
A Roman wheel-plow is illustrate1746.
A Roman wheel-plow is illustrated in the accompanying cut, and needs no particular description.
Busby's wheel-plow (English) received the Council Medal in its class at the London Exhibition of 1851.
It is mentioned in an English treatise as the best specimen of a plow that is manufactured.
It has land and furrow wheels, a skim colter, and is principally or altogether of iron.
1. A hydrostatic press for forcing car-wheels on to their axles and removing them.
The axle is suspended