278 AZIMUTH COMPASS 



described, in the same manner as with other glazes, and then fired in the porcelain 

 furnace. After this they aro covered with a brown glaze, which in reflected light 

 appears to bo filled with a countless number of light gold spangles. 



A thin fragment of the glaze appears, under the microscope, by transmitted light, 

 as a clear brownish glass, in which numerous transparent green six-sided prisms of 

 oxide of chromium, and some brownish crystals, probably of oxide of chromium and 

 peroxide of iron, are suspended. The oxide of chromium, therefore, separates on the 

 slow cooling of the glaze in the porcelain furnace, from the substance of the glaze 

 a silicate of potash, lime, and alumina, saturated with the peroxide of iron and 

 shines through the brownish mass with a golden colour. When the aventurine glaze 

 is mixed with an equal amount of colourless porcelain glaze, the glassy mass no 

 longer has a brown colour after the burning, but a light greenish-grey, and the 

 eliminated crystalline spangles likewise exhibit in reflected light their natural groen 

 colour. 



AVERRUNCATOR. A pair of pruning shears, which, on being mounted on a 

 polo some ten feet long, and actuated by a string of catgut, can be used for pruning 

 at a considerable distance above the head. 



AVOCADO-FEAR OX!L. An oil obtained from the oleaginous fruit, the Avocado 

 pear-tree (Persea gratissima), a native of Trinidad. A portion of this oil having been 

 submitted to Dr. Hofmann by the Governor of Trinidad, he reported on its character : 

 ' According to my present experience, the oil of the Avocado pear is less valuable 

 as a lubricating material. To make it tit for the higher classes of machinery, its 

 mucilaginous constituents must be removed by the same refining process requisite for 

 its adaptation in illuminating purposes, 



' On the other hand, the oil of the Avocado pear is very applicable for the pro- 

 duction of good soap. I have the honour of transmitting to your Excellency specimens 

 prepared with the oil : the smaller one, which possesses a yellow colour, is prepared 

 with the oil in its original condition ; the larger one is made with a portion of oil 

 which had previously been bleached by chlorine. From this specimen it is obvious 

 that the oil, although poor in stearine, nevertheless furnishes a soap which is tolerably 

 hard and solid. I have even now no hesitation in stating that, for the purposes of 

 the soap-maker, the oil of the Avocado pear will have, at least, the same value as 

 palm oil.' 



AXE. A tool much used by carpenters for cleaving and roughly fashioning 

 blocks of wood. It is a thin iron wedge, with an oblong steel edge, parallel to which, 

 in the short base, is a hole for receiving and holding fast the end of a strong wooden 

 handle. 



AXE-STONE. A sub-species of jade, found in Corsica, Saxony, and on the banks 

 of the Amazon. It is a silicate of magnesia and alumina, coloured by oxide of 

 chromium. See JADE. 



AXXNITE, called also Thumite. A silicate of magnesia, alumina, and iron, con- 

 taining boracic acid. It derives its name from the axe-like bevelling of its 

 lateral edges. This mineral is harder than felspar, and varies in colour from a 

 violet-brown to a leek-green. It is found in many parts of the Continent, and at 

 Botallack, St. Just, and at Trowellard, Cornwall, in fine brilliant clove-brown 

 crystals. 



AXliE-GREASE^ Several kinds of unguents employed to reduce the friction of 

 wheels circulating on their axles. See ANTI-ATTBITION. 



AXLES, of carriages. See WHEEL CABRIAGES. 



AXUNGE. Hog's lard. See FAT and OILS. 



A7R STONE, called also Scotch stone and snake-stone, is much in request as a 

 polishing stone for marble and for copper plates. These stones aro always kept 

 damp, or even wet, to prevent their becoming hard. 



The harder varieties of Ayr stone are now employed as whetstones. 



AZAXiE (from Azala, Arabic for madder). A colouring-matter obtained from 

 ' flowers of madder,' perhaps crude alizarine. It has been proposed to introduce azalo 

 in Franco as a dye-stuff. 



AZAXiEINE. A name for aniline-red. 



AZIMUTH COMPASS. The azimuth compass is used chiefly to note the 

 actual magnetic azimuth, or that arch of the horizon intercepted between the azimuth, 

 or vertical circle passing through the centre of any heavenly body, and the magnetic 

 meridian. 



The card of the azimuth compass is subdivided into exact degrees, minutes, and 

 seconds. To the box aro fixed two ' sights," through which the sun or a star may bo 

 viewed. The position into which the index of the sights must be turned to see it, 

 will indicate on the card the azimuth of the star. When the observations aro intended 

 to be exact, telescopes take the place of the sights. By this instrument we note the 



