756 CENTRIFUGAL MACHINE 



nir, affords a comont capable of repairing fractured bodies of all kinds. It requires a 

 few weeks to harden. When stone and iron are to be cemented together, a compound 

 of equal parts of sulphur and pitch answers very well. 



Lapidaries' cement is made of resin, tempered with bees'-wax and a little tallow, and 

 hardened with red ochre or Spanish brown and whiting. 



Opticians' cement, for fixing glasses for grinding, is made by mixing sifted wood ashes 

 with melted pitch, the essential oil of which is absorbed by the wood ashes, and the ad- 

 hesiveness of the pitch is therefore reduced. The proportions are somewhat dependent 

 on the temperature of the weather and the qualities of the pitch ; but generally about 

 4 Ibs. of wood ashes to 14 Ibs. of pitch are employed, and the cement, if too hard and 

 brittle, is softened with hogs' lard and tallow. 



Japanese cement is said to be prepared by mixing rice flour intimately with cold 

 water, and then boiling the mixture ; it is white, and dries nearly transparent. See 

 MOETAE. 



For Portland cement, Roman cement, Scott's cement, &c. See HYDHATOIC CEMENTS. 



CENT ATTRY. Centaurea calcitrapa. The common star-thistle ; an herb, the root 

 and leaves of which are bitter. The term centaury is also applied to Erythraa Cen- 

 taurium (the common centaury), one of the Gentian family. It is said to be sometimes 

 used instead of hops. 



CENTXAJ&E. See METRIC SYSTEM. 



CENTIGRADE SCALE. See THERMOMETER. 



CENTIGRAMME. See METRIC SYSTEM. 



CENTIMETRE. See METRIC SYSTEM. 



CENTNER. The Zollverein Centner contains 110*231 English Ibs. avoirdupois. 



CENTRIFUGAL MACHINE. A machine in which advantage is taken of the 

 force produced by centrifugal motion. Several of the old mills were of this descrip- 

 tion. This term has been not very correctly applied to Barker's Mill as it is often 

 called ; but which was fully described by Desaguiliers (vol. ii. p. 468), in which 

 water is made to act on a machine by its weight. Barker's mill consists of an/ 

 upright pipe which communicates with two horizontal branches having holes near 

 their ends opening in opposite directions. Desaguiliers affirms the pressure to be the 

 weight of a column, which would produce a velocity of efflux equal to the difference of 

 the velocity of the fluid and of the machine ; and hence he deduces that its performance 

 will be the greatest possible when its retrograde velocity is one-third of the velocity 

 acquired by falling from the surface, in which case it will raise ^ths of the water 

 expended to the same height, which is double of the performance of a mill acted on by 

 the impulse of water. 



But this is a very imperfect account of the operation. When the machine 

 (constructed exactly as we have described) moves round, the water which issues 

 descends in the vertical trunk, and then, moving along the horizontal arms, partakes 

 of this circular motion. This excites a centrifugal force, which is exerted against the 

 ends of the arms by the intervention of the fluid. The whole fluid is subjected to this 

 pressure (increasing for every section across the arm in the proportion of its distance 

 from the axis), and every particle is pressed with the accumulated centrifugal forces 

 of all the sections that are nearer to the axis. Every section, therefore, sustains an 

 actual pressure proportional to the square of its distance from the axis. This increases 

 the velocity of efflux, and this increases the velocity of revolution, and this mutual co- 

 operation would seem to terminate in an infinite velocity of both motions. But, on the 

 other hand, this circular motion must be given anew to every particle of water as it 

 enters the horizontal arm. This can be done only by the motion already in the arm, 

 and at its expense. Thus, there must be a velocity which cannot be overpassed even 

 by an unloaded machine. But it is also plain, that by making the horizontal arm very 

 capacious, the motion of the water from the axis to the jet may be made very slow, 

 and much of this diminution of circular motion be prevented. ' System of Mechanical 

 Philosophy, by John Eobinson,' pp. 616, 617. Centrifugal pumps, correctly speaking, 

 are such as have water admitted at the axis of a hollow wheel, traversed by vanes, 

 which being made to revolve rapidly expels the water at the circumference. The pipe 

 by which the water reaches the axis of the wheel or the reservoir which feeds it, 

 becomes under these circumstances a suction-pipe, and if the reservoir into which the 

 water is received from the periphery of the wheel be closed, and a pipe be carried from 

 it upwards, the latter becomes a force pipe. Appold's Centrifugal Pump, and Gwynn's, 

 are examples of this kind of machine in practice ; they arc largely employed in raising 

 large bodies of water to a small height. Centrifugal machines were used in America 

 in 1830 ; Mr. M'Carty erected one in the navy yard, Now York, in 1830. M. Ch. 

 Combes describes a centrifugal machine in a paper entitled ' Sur Us Eoues de Reaction,' 

 (Comptes Eendus, vol. vii. 2me. Semestre, 1838, p. 306). In this paper the theory of 

 the centrifugal ventilator is discussed, and its obvious relations to the theory of the 





