HOR 



HOR 



the Labrador, the basaltic, and the horn- 

 blende slate. The common hornblende 

 is of a greenish black, or raven-black, 

 which in some varieties approaches to a 

 greyish and even velvet black. It occurs' 

 in mass, disseminated and crystallized. Its 

 specific gravity is from 3.6 to 3.8. It is 

 opaque, gives a greyish streak, is mode- 

 rately hard, and easily frangible; it ex- 

 hales an argillaceous odour. It melts be- 

 fore the blow-pipe into a greyish black 

 glass; the constituent parts, according to 

 Kirwan, are, 



Silica . . . 37.0 



Alumina . . 

 Magnesia . . 

 Lime ... 

 Oxide of iron 



22.0 



16.0 



2.0 



23.0 



100.0 



The common hornblende forms one of 

 the essential ingredients of several moun- 

 tain rocks; is sometimes accidentally dis- 

 seminated in others, and occurs in beds. 

 When in beds, it is frequently accompa- 

 nied with ores, as magnetic iron-stone, 

 iron pyrites, &c. It is found in all the 

 northern parts of Europe ; and when pure 

 is a capital flux for iron ores, to which 

 purpose it is applied in Sweden, where it 

 is obtained in large quantities. 



The Labrador hornblende is found in 

 the island of St Paul, on the coast of La- 

 brador, is usually of a brownish black, and 

 its specific gravity is 3.S8. The horn- 

 blende slate is of a colour intermediate 

 between greenish and raven black ; it is 

 massive, and is generally mixed with mica 

 and felspar. It occurs in beds of primi- 

 tive rocks, particularly in clay slate ; also 

 in gneiss and mica slate, and is found 

 principally in the northern "parts of Eu- 

 rope. The basaltic hornblende is of a 

 velvet b!r*ck, occurs almost always in sin- 

 gle imbedded crystals, which are small 

 and middle sized. The surface is smooth 

 and shining, except where it happens to 

 be covered by a thin ochery crust. The 

 specific gravity is rather less than that of 

 the hornblende slate. It melts before 

 the blow-pipe into a brownish black glass. 

 Berg-man has analyzed it, and found it 

 contained. 



Silica .... 58.0 

 Alumina . . . 27.0 

 Iron .... 9,0 

 Lime .... 4.0 

 Magnesia ... 1.0 



Loss 



VOL. VI. 



[99.0 

 . 1.0 



100.0 



It is found in Saxony, Bohemia, Scot- 

 land, Italy, &c. It resists decomposition 

 longer than basalt, hence crystals of it are 

 found in clay, formed by the decomposi- 

 tion of basaltic rock. Common hornblende 

 is difficultly frangible, but the basaltic is 

 very easily frangible. 

 HORNET. See VESPA 

 HOROLOGY. Horology is that branch 

 of science which enables us to measure 

 the portions of time. We judge of the 

 lapse of time by the succession of sensi- 

 ble events ;and the most convenient and 

 accurate measures of its quantity are de- 

 rived from motions, either uniform, or 

 else repeated at equal intervals. Of the 

 former kind, the rotation of the earth on 

 its axis is the most exact, and the situa- 

 tion of its surface with regard to the fix- 

 ed stars, or, less simple, with regard to 

 the sun, constitutes the means for deter- 

 mining the parts of time as they follow 

 each other. See ASTRONOMY and DIAL- 

 UJTG. Of the latter kind, the rotation of 

 machinery, consisting of wheel-work, 

 moved by a weight or spring, and regulat. 

 ed by a pendulum or balance, affords in- 

 struments of which the utility is well 

 known. The term horology is at present 

 more particularly confined to the princi- 

 ples upon which the art of making clocks 

 and watches is established. A considera- 

 ble portion of this extended subject of 

 research has been given under the articles 

 CLOCK and CHRONOMETER. In the present, 

 we shall chiefly attend to the means by 

 which the train of wheel-work is made to 

 make a number of successive advances, 

 all so very nearly equal in the measure- 

 ment of time, that a surprising degree of 

 precision is obtained in ascertaining the 

 intended object. 



The machines which, for centuries, 

 have been commonly used to measure 

 time, consist of a movement, or train of 

 wheels, drawn by a weight or spring, and 

 a regulator, the object of which is to 

 keep the motion of the train within the 

 required degree of uniformity. A conti- 

 nual rotatory motion, which constantly 

 tends to accelerate, is thus corrected by 

 means of an alternate motion ; while the 

 power which carries round the move- 

 ment restores also, to the regulator, the 

 action lost by friction and other causes. 

 The mechanism, by which the two prin- 

 cipal parts act on one another, is called 

 the escapement; and this most admirable 

 contrivance may be reckoned the distin- 

 guishing characteristic of the modern art 

 of time-piece making. 



One of the most ancient escapements is 

 that which is now applied in almost all 

 Y 



