491 



water liunl, and such water i-, useful in the brew- 



'I some kind- <!' lieer. l'iin-1 /nin/i'iii/ii/, used 



tilling in tin- manufacture of gome kinds of 

 paper, i- .in artiiicial sulphate of lime, precipitated 



D) sulphuric acid I'roiu chloride of calcium. I-'irttlf 

 /I-/,/-;/ is planter of Paris which has lieen made to 

 al>-oilt beeswax, spermaceti, and stearic acid, in 

 their melted Mate. 'I'lie average annual production 

 of gypsum in (I real Britain is nearly 1 'JO, 000 tons, 

 value about tls,(HMi. In 1888 the' production of 

 the Tailed States was about 96,000 tons ; of Nova 

 Scotia, 1*2(5, 1 1H tons. For the anhydrous sulphate 

 <>t lime, see .\\in HIM 1 1 . 



<)'|sy-wort ( I'l/ni/iHti I'lo'opccus), sometimes 

 alto called Water Horehound, is a perennial plant 

 belonging to the natural order Labiatie. It is a 

 tall ereet branching plant, slightly hairy, with a 

 creeping root-stock. It is common in moist places 

 in lit it. -i in. the Continent, Russian and central 

 AM:I, and North America; and is regarded as a 

 I'elirifnge and astringent. It dyes black, and gives 

 a permanent colour to wool, linen, and silk, and as 

 long ago as 1578 the Gypsies were fabled to stain 

 their skin with it. The Bugle-weed of North 

 America (L. virginiCua) has more powerfully 

 astringent properties. 



Gyrfalcon. See FALCON. 



Gyroscope (Greek) is the name given to an 

 instrument for the exhibition of various properties 

 of rotation and the composition of rotations. It 

 differs from a top in having both ends of its axis 

 supported. The invention is probably French or 

 German, and in some of its forms it dates from 

 al>out the end of the 18th century. 



If a mass be set in rotation about its principal 

 axis of inertia of greatest or least moment, it will 

 continue to revolve about it ; and, unless extraneous 

 force be applied, the direction of the axis will remain 

 unchanged. Such, for instance, would be the case 

 with the earth, were it not for the disturbances (see 

 NUTATION and PRECESSION) produced by the sun 

 and moon : the direction of the axis would remain 

 fixed in space. It is for this very reason that modem 

 artillery is rifled. If, then, a mass of metal, as, 

 for instance, a circular disc, loaded at the rim, and 

 revolving in its own plane, be made to rotate rapidly 

 about its axis of greatest moment of inertia, and if 

 it be freely supported (in gimbals, like the box of a 

 compass), the direction of its axis will be the same 

 si> long as the rotation lasts. It will therefore con- 

 stantly point to the same star, and may, of course, 

 be employed to show that the apparent rotation of 

 the stars about the earth is due to a real rotation of 

 the earth itself in the opposite direction. This 

 application was made by Foucault shortly after his 

 celebrated Pendulum (q.v.) experiment, as it had 

 been many years before (March 1836) by Dr 

 Sang (see the Trans, of the R. Scot. Soc. of Arts). 

 It is, in practice, by no means so perfect a mode 

 of proving the earth's rotation as the Foucault 

 pendulum ; but this arises solely from unavoidable 

 defects of workmanship and materials. Professor 

 I'iax/i Smyth has applied this property of the 

 gyroscope to the improvement of our means of 

 making astronomical observations at sea. A tele- 

 scope, mounted on the same support as the ends 

 of the axis of the gyroscope, Mill, of course, be 

 almost unaltered in position by the rolling or 

 pitching of a vessel ; and a steady horizon, for 

 sextant observations of altitude, may be procured i 



by attaching a mirror to the support of the gyro* 

 scope, and setting it once for all by means of Hpirit- 

 levels. 



But the most singular phenomena shown by the 

 gyroscope are tho-e depending on the roinponition 

 of rotations (see ROTATION). Any motion what- 

 ever of a body which has one point fixed in of the 

 nature of a rotation about an axis passing through 

 that point. Hence, simultaneous rotations about 

 any two or more axes, being a motion of wmie kind, 

 are equivalent to a rotation about a tfingle axis. 

 The effect, then, of impressing upon the frame in 

 which the axis of the spinning gyroscope in sus- 

 pended a tendency to rotate about some other axil*, 

 is to give the whole instrument a rotation about an 

 intermediate axis ; and this will coincide more nearly 

 with that of the gyroscope itself, as the rate of it . 

 rotation is greater. The compound motion con- 

 sists in the rolling of an imaginary cone fixed 

 in the gyroscope upon another fixed in space ; 

 the rotation of the axis of a top rounu the 

 vertical (when it is not 'sleeping' in an upright 

 position), and the precession of the earth's axis, 

 are precisely similar phenomena. Thus, when 

 the gyroscope is spinning, its axis being hori- 

 zontal, a weight attached to the framework at one 

 end of the axis ( fig. b ) makes the whole rotate about 

 the vertical ; attached to the other end, the rotation 

 takes place in the opposite direction. And the 

 framework may be lifted by a string attached near 



Gyroscope. 



one end of the axis (fig. a ) without the gyroscope's 

 falling. Its axis still projects horizontally from the 

 string, but it revolves as a whole round the string. 

 Various other singular experiments may be made 

 with this apparatus ; and others, even more curious, 

 with the gyrostat of Sir W. Thomson, which is 

 simply a gyroscope enclosed in a rigid case, by 

 which the ends of its axis are supported. When a 

 gyrostat is made the bob of a pendulum under 

 certain conditions, the plane of vibration of the 

 pendulum turns, as in Foucault's celebrated experi- 

 ment, but in general at a much greater rate. 



