MlNKKALOiJY. 



tboir way to Now York, instead of going, u they call it, straight 



acroKH tho Atlantic , :m 1 they wonder why their passengers 



Hliould talk about icebergs encounter .1 study of a 



I..IM- will di.s|>i-l tlic.^o illusions, and show that the 



Mid and Now York actually 



HO high that at all seasons of tho year it 



ho followed. Apply tlm mune length of thread in the 



1 by tlm 'nppiin'ntlyi .-tr:iij,'lit course OU tho chart, 



ami it will full short by some hundreds of miles. 



For all this, groat circle sailing has boon Jitt:.< ])racti8od until 



.ui;h it is easy enough to find tho groat circle 



'.aces, it is IOSH cosy to htoor it, aa the 



s course is always changing, owing to tho course a 



t li-in in it-i relation t> tho earth's eurfaco cutting 



us at different angles. Tho calculation 



involved in ascertaining the incessant but varying change of 

 compass course until lately barred tho way against ordinary 

 navigators. Now, however, tables drawn up by Mr. Towson, of 

 Dovonport, arc published by tho Admiralty, based on the prin- 

 ciple of breaking up tho aro into short rhumb-lino stages equal 

 to ono degree of longitude. The tables show tho change of 

 course to be adopted at each stage ; thus tho sailor keeps up 

 liliar and, it must bo owned, simple rules of sailing, and 

 yot the great circle, though not exactly, is tolerably followed. 



Of course when a ship sails on tho old system on a meridian, 

 or on tho equator, she is practising great circle sailing without 

 probably intending it. 



\y<- have now finished our exposition of tho mathematical 

 principles and processes by which tho mariner traces his vessel's 

 track upon the ocean, or keeps his " account by dead reckon- 

 ing." It has been stated that the data from which ho calcu- 

 lates are themselves greatly liable to error, and in view of this 

 it may bo thought we have bestowed unnecessary exactness 

 upon results which can never bo taken without question. In 

 practice, too, there is no doubt that many things wo have made 

 Bubjocts of calculation are simply guessed by tho mariner 

 with results usually accurate enough for the purpose, if he be up 

 to his work. This is true, but nevertheless there is no ground- 

 work for practice so sure as a good knowledge of theory, and 

 even tho practical work of guessi~q results is best done by those 

 who understand how to evolve them with scientific accuracy. 



Tho science of Nautical Astronomy or rather tho art of 

 using the instruments hy which longitude and latitude are 

 found at sea by celestial observations is too technical and 

 limited in its character to find place in elementary lessons 

 like these, which seek only to apply to navigation those mathe- 

 matical principles of general utility which have been taught in 

 other sections of this work. But just as tho mariner must 

 learn the practical work of observing 8>>eed and course, currents 

 and leeway, and the use of the observing instruments, tho log 

 and compass, so must ho learn to use sextant and quadrant 

 a check upon his own estimates and calculations. 



MINERALOGY. VII. 



NITRATES. 



ONLY two important nitrates appear in Nature. 



Nitrate of Soda, or Chili saltpetre. This mineral is fonnc 

 in many places on the coast of Chili. At Tarapaca, 3,300 

 feet above the sea, is a vast deposit of several feet thick 

 and it bears every indication of having a marine origin. It is 

 said that tho natives build their huts of blocks of this salt 

 proving the absence of rain in the district, as nitrate of soda is 

 soluble in water. It crystallises in rhombohedra. When 

 thrown on live coal it causes a vivid combustion, tho soda im 

 parting its characteristic yellow tinge to tho flame. It would 

 be used largely for making gunpowder, but it deliquescent 

 that is, it imbibes water from tho air. 



Nitrate of Potash, or saltpetre, is peculiarly valuable as 

 being the chief ingredient in gunpowder. It is frequently 

 found in caves of limestone districts, and in India and other 

 countries appears as an efflorescence on the soil in tho hot 

 weather which succeeds copious rains. In Norway and Sweden it 

 is artificially prepared by exposing refuse heaps to the action of 

 tho air. At tho end of three years the "nitre bed" is lixiviated 

 that is, washed with water. This water on evaporation yields 

 crystals of saltpetre in long thin prisms. The salt u trimctric. 



OABBOMATM. 



Carbonate oj Limt.Thi* mineral is very widely 

 in Nature, ainco of it aro <x>mpoed aU limestone aad chalk ruck*. 

 When it appear* In a crystalline form, it is M ft rbosabobertroai 

 of the hexagonal nytom a figure which is the liMiiJMiliil torn 

 Df the icoaahedron (oo page 170). When crystallised it if 

 ailed Cale Hpar, and if the crystals are transparent it is 

 termed Iceland Spar. Tho fact that it possesses the property 

 of double refraction has boen alluded to and illustrated in pag 

 49. Its constitution in, 1 atom of carbonic add and 1 at Una. 



Ara<joiiite baa precisely tho aame competition mm eale par. 

 Some specimens contain a little carbonate of atroatta; bet 

 .bis is not an essential ingredient. It crystallises, however. 

 n tho trimctric system ; bonce carbonate of lime i* aaid to be 

 dimorphous. Aragouito U of a wine-yellow colour; iU nj stele 

 aro usually clustered and radiating. It u frequently associated 

 with gypsum, is found in the fossil bclemnite, and it is aaid 

 that cold springs containing carbonate of lime deposit calo spar, 

 while hot springs yield aragonito. It U named from Aragon, is 

 Spain. Carbonates of lime effervesce when touched with add. 



Several of the ores of tho metals appear ae carbonates. 



Clay Ironstone is found in beds in the Carboniferous system. 

 It is a carbonate of iron, and is tho chief source of that useful 

 metal in England. 



fyathic Iron Ore is the carbonate of iron when crystallised, 

 and appears in tho hexagonal system in rhombohedrons and six- 

 Hided prisms ; frequently tho faces aro curved. When foliated 

 and massive it is called Sparry iron. Its colour is light greyish 

 to brown, and the scales aro often translucent. 



Calamine, carbonate of tine, is the most valuable ore of 

 that metal. Calamino has silica usually present ; but when 

 only the oxide of zinc and carbonic acid are tho constituents of 

 the mineral, it is termed Zinc Spar. 



Malachite is carbonate of copper. It contains 1 atom of 

 carbonic acid, 2 of oxide of copper, and 1 of water. It is 

 found on the walls of lodes as a stalactitic growth, and the 

 well-known appearance of the green stone at once indicates 

 that it was enlarged by aggregations ; it U most probably the 

 result of tho action of water containing carbonic acid on decom- 

 posing sulphurot of copper (copper pyrites). 



Azurita is blue, and contains 2 atoms of carbonic acid, 3 

 of oxide of copper, and 1 of water. It is frequently found 

 associated with malachite. The largest deposits of the Utter 

 mineral were in tho Ural Mountains, and because it was suscep- 

 tible of a high polish tho stone was in great favour for all 

 species of ornamental work, and always commanded a high 

 price ; but latterly such enormous quantities hare been dis- 

 covered in Australia that its value is much deteriorated. 



Cerusite is carbonate of load, which is sometimes found with 

 galena, to whose decomposition it is due. 



PHOSPHATES. 



Apatite is phosphate of lime. It is never pure, but the 

 fluoride and chloride of lime are invariably associated with it 

 in small quantities. When found crystallised it is in short 

 six-sided prisms of a greenish colour. It is found in many 

 metamorphic rocks, and in some granites. When present in 

 nodules in stratified rocks it is accounted to bo of organic origin, 

 for phosphate of limo is the chief constituent of bones. 



Wavellitc is a hydrated phosphate of alumina ; it ia uroi 

 found as small hemispheres which have a finely radiated 

 ture within, attached to tho surface of tho rock, generally of a 

 greenish shade of colour. When forced off, they leave a stellate 

 circle on the rock. 



Turquoise is also a hydrated phosphate of alumina. 

 usually contains a trace of copper. It is valued as a gem. 

 is found amorphous, occurring in veins in the mounts***] 

 districts of Persia. It is capable of a fine polish, and loses its 

 beautiful blue colour in muriatic acid. 



Vivianite is a hydrated phosphate of iron. I ;: ;*- 



blue colour, and is found with bog iron ore. 

 in the interior of fossils, and in veins traversing clay slate. 



BORATKS. 



Wo have already soon that boracio acid enters into the com- 

 position of tourmaline and axinite, but only as a subordinate, 

 not a prominent ingredient. There are two minerals, however 

 which are formed by this acid united to a base, 



