704 



TURNINGTURNIP. 



yields to the knife, affording a grayish-white pow- 

 der. It contains illumine, lime, magnesia, and very 

 little iron. Its only locality is mount Sorel, in 

 Pauphine, where it occurs accompanied by quartz, 

 f-M| ar and anatase. 



TUttNING, in mechanics ; a very ingenious mid 

 useful art, by which a great variety of articles are 

 manufactured, by cutting or fashioning them while 

 they revolve upon an axis or line, which in most 

 cases remains immovable. Every solid substance 

 in nature may be submitted to this process ; and, 

 accordingly, we have articles turned in the metals, 

 in wood, in pottery, in stone, in ivory, &c., so 

 numerous, and so universally in use, that it would 

 be superfluous to point them out. The simplest 

 process of turning is that of the potter, who, in the 

 first stage of forming his ware, sticks a piece of wet 

 clay upon a wheel, or flat table, while it revolves 

 horizontally, and, in this state of rotation of the 

 clay, fashions it, with the greatest facility, into 

 vessels of every description. But in most opera- 

 tions of the art, the revolving body is cut or shaved 

 by applying a chisel, or other suitable tool, to its 

 surface, while in motion ; a condition that requires 

 firmness in the axis of rotation, and also that the 

 tool itself should be steadily supported. The in- 

 strument or apparatus for these purposes is called a 

 lathe. Among the great varieties of lathes, it is 

 indispensably required, for circular turning, that the 

 work should be supported by two steady centres, 

 or by parts equivalent to two centres, at a dis- 

 tance from each other in the axis of rotation, and 

 that the tool should be supported by a steady bar, 

 or a piece called the rest. A great number of 

 turned articles either have, or will admit of a per- 

 foration through their axis. All wheel-work, and 

 most of the articles turned in wood, are of this 

 description. Clock and watch-makers accordingly 

 use a very cheap, simple and portable lathe, called 

 a turn-bench, consisting of a straight bar of iron, 

 about five inches long, with two cross bars or heads, 

 about two inches long, one fixed at the end of the 

 long bar, and the other capable of being shifted by 

 means of a socket and screw. In each of these 

 heads is a centre-pin, terminating in a point at one 

 end, and in a central hole at the other, like the 

 centre-pin in the poppet-head of any other lathe ; 

 the use of which is to afford point-centres when the 

 points are turned towards each other, or hole-cen- 

 tres when the contrary is the case ; and lastly, there 

 is a small rest, with its support, slidable and ad- 

 justable ajong the bar, as in another lathe. These 

 instruments will therefore support any piece of 

 four or five inches long, and three inches diameter, 

 between the centres ; and the method of producing 

 the rotation is by passing the catgut string of a bow 

 once or twice round the work, and drawing the 

 bow backwards and forwards with one hand, while 

 the other is employed in applying the tool. The 

 turn-bench itself is held steady in a vice fixed to a 

 bench or stand. The common lathe of the turners 

 in wood, called the pole-lathe, is the same thing as 

 the watchmaker's turn-bench, but upon a large 

 scale, and a little varied. Instead of the horizontal 

 bar, it has two long stout bars of wood, called 

 shears, forming what is called the bed of the lathe, 

 and its two poppet-heads are upright blocks of 

 wood, mortised in between the shears, above which 

 they rise and carry the centre-screws, and between 

 which they are movable, and may be wedged firmly 



upon a wooden mandrel, and is made to revolve nj 

 a string or bend, proceeding from a long spinning 

 pole at the ceiling or roof of the shop, round the 

 work, and thence to a treadle or foot-boani, which 

 acts by alternate pressure from the foot, while the 

 workman applies the cutting tool with his liamK 

 In these, and all similar lathes, the rotation is made 

 backwards and forwards; and there are some kinds 

 of work in which such a motion is advantageous; 

 but in general it is much preferable that the work 

 should constantly revolve the same way as in the 

 lathe usually known by the name of the foot-luiln'. 

 The stronger, the firmer, and the better the work- 

 manship of a lathe, the easier it will be to perform 

 work with expedition and truth ; but a good work- 

 man will make true and excellent work with a verv 

 indifferent lathe, by taking care to cut so little at a 

 time, that the parts of the engine may never In- 

 shaken out of their contact. Metallic latli 

 ever so strong, have an elastic tremor, which r 

 it difficult to cut brass and bell-metal as firmly and 

 smoothly as in wooden lathes; but the structure 

 of the former admits of greater precision and truth. 

 The velocity of rotation may be extremely swift in 

 wood, slower in brass and bell-metal, still slower 

 in cast-iron, and slowest of all in forged iron or 

 steel. The reason for these limits appears to be, 

 that a certain time is requisite for the act of cutting 

 to take place, and that the tool itself, if heated by 

 rotation, will instantly become soft, and cease to 

 cut. Steel and iron require to be kept wetted. 

 Hitherto we have spoken of plain turning, which is 

 indeed the most useful and most universally prac- 

 tised. But many other nice and very curious ope- 

 rations are performed by this art. If the poppet- 

 heads, supporting the mandrel, be made regularly 

 to move from side to side, during the rotation, or 

 the rest be made to approach to, and recede from, 

 the work, any number of times in a turn, the cuts 

 will not be circular, but undulating, indented or 

 waved in any curve that may be required. Work 

 of this kind, which is chiefly done on watch-cases, 

 snuff-boxes and trinkets, is called rose-work. The 

 motion is commonly regulated by certain round 

 plates of brass fixed on the mandrel, called roves, 

 which have their edges waved, and are called roses. 

 The act of turning is so extensively applicable, that 

 it would require a volume to describe its uses, and 

 the methods of practising it. The largest columns, 

 the most ponderous artillery, and the minutest pivots 

 of watch-work, with all wheeUwork, rotatory ma- 

 chines, vessels, &c,, are worked in this method. 



TURNIP (brassica rapa) ; a cruciferous plant, 

 belonging to the same genus with the cabbage, ex- 

 tensively cultivated for the sake of its esculent root. 

 This latter is turbinate, more or less depressed, but 

 varies somewhat in colour, size and form in the 

 subvarieties produced by culture. It is of a fleshy 

 consistence, and has a sweet, somewhat pungent, 

 and agreeable taste. The radical leaves are oblong 

 and lyrate ; the upper ones entire ; the flowers are 

 usually yellow. The smaller varieties, in general, 

 are most agreeable to the taste, and most esteemed ; 

 but the quality depends very much on the nature of" 

 the soil, which should be sandy and light. The 

 ordinary season of sowing is from the end of June 

 to the beginning of August ; but if it is desired to 

 procure them throughout the season, they may be 

 sown from March till September. Turnips are a 

 wholesome article of food, much in use. The large- 



at any required distance from each other. The rooted varieties have been employed in Europe for 

 work itself is either put between the centres, or | fodder, during the winter season, from time imme- 



