Ml 



GYPSUM. 



OYROSCOI'K. 



I : 



eooceming thU strange race, in hi* ' Historical Surrey of the Customs, 

 HabiU, and present State of the Gypsies ; designed to develop.- the 

 Origin of thi* Singular People, and to promote the Amelioration of 

 their Condition,' 8vo, York, 1816. He hat largely made use of the 

 work of Grellman ; but for a correct account of the peculiar manner*, 

 language, and probable descent of the race, the boat authority in 

 Mr. G. Borrow, particularly hia ' Zincali ; or an Account of the Gypsies 

 of Spain.' 



GYPSUM. The hydrous sulphate of lime furnishes materials which 

 are extensively used in building operations, cither for the purpose of 

 obtaining platter of Pa rit from the amorphous varieties ; or for orna- 

 mental sculpture, when the gypsum assumes a sub-crystalline anhydrous 

 character, known by the name of alabaster. The amorphous gypsum 

 is raised in enormous quantities in part* of Lincolnshire and in Derby- 

 shire, in the neighbourhood of Paris, in Provence, Tuscany, north of 

 Spain, lower Austria, Nova Scotia, &c. The anhydrous varieties are 

 principally obtained from the departement of the Isere in France, 

 Volterra in Tuscany, and the Guadalajara in Spain. The gypsum of 

 the tertiary deposits of France, it may be added, contains a notable 

 proportion of carbonate of lime, and even of soluble silica ; and it is no 

 doubt on this account that it is able to be employed in external 

 masonry, which is not the case with the English plaster. Another 

 peculiarity of the real plaster of Paris is that it swells with inconceivable 

 force in setting, and therefore requires to be used with great caution. 



Some artificial plasters are made, under the names of Keenc's, 

 Martin's, and Parian cements, by slacking the dehydrised sulphate of 

 lime with solutions of borax, alum, or other salts, and then exposing it 

 to a second calcination. 

 GYPSUM. [CALCIUM, Lime, Sulphate of.] 



GYRATION, CENTRE OF. When a system of heavy bodies, or 

 any system possessing weight, has a fixed axis of revolution, the centre 

 of gyration is a point at any such distance from the axis, that the 

 moment of inertia would not be altered if the whole mass were collected 

 at that point. The moment of inertia being found by multiplying 

 every mass by the square of its distance from the axis, the distance 

 of the centre of gyration is found by dividing this moment of inertia 

 by the whole mass, and extracting the square root of the quotient. 

 As this term is now very little used, we refer to INEHTIA for further 

 information. 



GYROPHORIC ACID. [LICHENS, COLOURING MATTERS OP.] 

 GYROSCOPE (from the Greek -yvpot, a circle or rotation, and 

 (TitoWw, to perceive), an instrument recently suggested by M. Foucault, 

 and improved by various modifications, and which has acquired 

 notoriety from its supposed efficiency in rendering riribte, by ita direct 

 dynamic effects, the diurnal rotation of the earth on ita axis. It seems 

 to have been originally invented in the form of jig. 1, by M. Fessel of 

 Cologne, and was described in Poggendorff's ' Annalen ' for September, 

 1853. It has since been modified and improved by Professor Pliicker, 

 and Mr. Wheatstone, and although originally invented apparently without 

 any knowledge of Bohnenbcrger's apparatus, is merely a modification 

 of it, as described in Gilbert's ' Annalen ' (Ix. p. 60). The essential 

 parts of Bohnenberger's apparatus were a sphere capable of rotating 

 about an axis whose extremities rest in opposite points of a hoop 

 which can turn on pivots horizontally, within another hoop turning on 

 pivots about a vertical axis. 



The principle on which the action of the instrument depends was 

 discovered by Frisi about 1750, and enunciated as the principle of the 

 ' composition of Rotatory Motions ; " and the instrument itself is 



Fig. 1. 



designed to exhibit experimentally the actual competition of rotations 

 about different axes impressed at once on the same body. \V- 

 know that if a body be set in motion in one direction, ami any force 

 trad to make it move in another direction, it will move in neither, but 



in an intermediate direction. So also we have the strictly analogous 

 ease of rotatory motion, namely, when a body is rotating about an 

 axis, and any force tends to make it rotate about another axis, it will 

 not rotate about either, but about a new axis intermediate to the other 

 two. Thus, to take a very simple, but paradoxical case (given by 

 Professor Baden Powell, in a paper read at the Royal Institution, in 

 March, 1854) : a wheel at one end of an axis, and a weight at the 

 other, are suspended in equilibrio; which is, of course, unaltered, 

 whether the wheel be at rest or in rotation ; the weight is then slid 

 so that the balance is destroyed ; now if the wheel be aet in rapid 

 rotation, the equilibrium is restored. 



The apparent displacement of the plane of vibration of the pendulum 

 was first noticed by the Academicians del Cimenlo. This fact was 

 brought to light in 1851, by Signor Antinori, director of the Museum 

 at Klon-iK . \vh.. found an autograph manuscript on the subject by 

 Vincento Viviani ; this note however was not published until aficV 

 Foucault's discoveries, who was the first to connect this fact with tin- 

 rotatory motion of the earth. M. Foucault communicated his dis- 

 covery to the Academy of Sciences at Paris, on the 3rd of February, 

 1851. His experiment with the pendulum will be fully described 

 under the article PKXDri.rsi, while we shall confine ourselves h 

 his later invention, the gyroscope, for showing the same rotatory 

 motion of the earth. We shall, however, first mention M. Liouville's 

 connection between this rotatory motion and the displacement of the 

 plane of rotation of the pendulum or disc of the gyroscope. The rota- 

 tion of the earth on iti a.n has been proved by Foucault (1) by his 

 celebrated pendulum experiment, (2) by his gyroscope. (1) Suppose 

 a pendulum suspended over either pole of the earth, and set in 

 oscillation. It is evident that a spectator, carried round by the 

 rotation of the earth, would pass, first under one end of the arc of the 

 vibrating pendulum, and then under the other, so that ita plane of 

 oscillation would appear to him to make a revolution from etut to tml 

 in the same time as the earth revolved from irtt to etut. (2) Let the 

 pendulum be similarly suspended over the equator. It is evident that 

 in this case no change of the plane of oscillation can take place, because 



Fig. 2. 



the spectator cannot, by being carried round, approach nearer to one 

 end of the arc of vibration than the other. (3) If tin- ]'iiilnhini in 

 suspended at any intermedia!' In this case, the rotation of 



the earth round the polar axis may be considered as the result 

 two rotations, one round an axis pawing through the place of observa- 

 tion, and another round a perpendicular axis. But the rotation round 



