ORDNANCE

, are all sorts of great guns, used in war; such as cannon, mortars, howitzers, &c.

ORFFYREUS's Wheel, in Mechanics, is a machine so called from its inventor, which he asserted to be a perpetual motion. This machine, according to the account given of it by Gravesande, in his Oeuvres Philosophiques, published by Allemand, Amst. 1774, consisted externally of a large circular wheel, or rather drum, 12 feet in diameter, and 14 inches deep; being very light, as it was formed of an ass<*>mblage of deals, having the intervals between them covered with waxed cloth, to conceal the interior parts of it. The two extremities of an iron axis, on which it turned, rested on two supports. On giving a slight impulse to the wheel, in either direction, its motion was gradually accelerated; so that after two or three revolutions it acquired so great a velocity as to make 25 or 26 turns in a minute. This rapid motion it actually preserved during the space of 2 months, in a chamber of the landgrave of Hesse, the door of which was kept locked, and sealed with the landgrave's own seal. At the end of that time it was stopped, to prevent the wear of the materials. The professor, who had been an eye-witness to these circumstances, examined all the external parts of it, and was convinced that there could not be any communication between it and any neighbouring room. Orffyreus however was so incensed, or pretended to be so, that he broke the machine in pieces, and wrote on the wall, that it was the impertinent curiosity of professor Gravesande which made him take this step. The prince of Hesse, who had seen the interior parts of this wheel, but sworn to secresy, being asked by Gravesande, whether, after it had been in motion for some time, there was any change observable in it, and whether it contained any pieces that indicated fraud or deception, answered both questions in the negative, and declared that the machine was of a very simple construction.

ORGANICAL Description of Curves, is the description of them upon a plane, by means of instruments, and commonly by a continued motion. The most simple construction of this kind, is that of a circle by means of a pair of compasses. The next is that of an ellipse by means of a thread and two pins in the foci, or the ellipse and hyperbola, by means of the elliptical and hyperbolic compasses.

A great variety of descriptions of this sort are to be found in Schooten De Organica Conic. Sect. in Plano Descriptione; in Newton's Arithmetica Universalis, De Curvarum Descriptione Organica; Maclaurin's Geometria Organica; Brackenridge's Descriptio Linearum Curvarum: &c.