430 O 
3nd that which remains in the retort acquires a blackifh 
tinge; empyreumatic, acetous, andfebacic, acids manifeft 
tlvemfelves, together with carburetted hydrogen and car¬ 
bonic acid of a very offenfive odour. Hence it is inferred 
that there is a real difference between animal and vegeta¬ 
ble oils, though it has not been pointed out by chemical 
analyfis. The coarfer kinds of animal oil, extra&ed by 
putrefa&ion and a ftrong heat, pofl'efs a much more dis¬ 
agreeable odour than any of the vegetable oils ; and, 
when rancid, difengage ammonia by the affion of the 
fixed cauflic alkalies, in which they alfo differ from the 
latter. The fifh-oils, always rancid, are for the moll part 
thick and glutinous, which renders them in forhe degree 
unfit for burning, and forne other ufes to which they are 
applied. Many attempts have been made to meliorate 
them ; lome of which we fhall notice by-and-bye. 
2. Vegetable oils, fuch as olive-oil, arid fome other kinds 
the produce of the fouth of Europe, are exprefled by a 
common ferew-prefs, provided with a windlafs or capftan. 
The olives are firft pounded, or bruifed, either in a large 
mortar, or by a running (tone, in the fame manner as 
the apples for makingeider. The pulp thus produced is 
put up in bags made of horfe-hair or rufhes; and, a pile of 
thefe being made up under the prefs, the Screw is forced 
down by men working at a long lever, and the oil ex¬ 
prefled : it runs very freely at firft ; and this, which is ef- 
teemed the beft quality, is in fome countries kept fepa- 
rate. When this preffurehas continued an hour or two, 
the power of the capftan is applied. This produces a good 
quantity of oil of the fecond quality, which is fold at an 
inferior price; and in fome provinces of Spain, where 
the olive is extenfively cultivated, it is ufed to burn in 
their lamps. 
A fecond mode of extracting oil is by immerfion in hot 
water. In this latter cafe, the oil feparates from the other 
ingredients with which it is naturally mixed, and riles 
by the force of gravity to the furface of the water, from 
which it is fkimmed off. Recently-drawn oil is more or 
lefs impure, on account of its containing a variable pro¬ 
portion of mucilage, fecula, and perhaps other lub- 
ftances : of thefe a part is always depofited by reft, efpe- 
cially if the contact of the air is not wholly excluded ; 
but another portion remains in permanent folution; and 
to this that partial fpontaneous d.ecompofition in oils, 
called “ rapeidity,” is principally owing. 
But for extracting the oil from linfeed, rapefeed, See. 
it is neceflary to apply the power of a mill. Thefe grains 
are exceedingly hard and finooth on their furfaces; and 
the fragments of their (hells, however broken, form lit- 
tie'Cavities which will retain the oil, unlefs a far greater 
preflu re is applied than can be obtained by a ferew-prefs ; 
it is therefore done by a wedge-prefs. This confifls of 
a : ftrong block of wood, or a caft-iron frame, in which, a 
long mortife is made: in this a bag of bruifed feed is 
placed at each end ; and, blocks being put in to fill up the 
mortife, a wooden wedgeis introduced between the blocks, 
and driven in by repeated blows of a heavy Hamper, 
which is raifed up by the power of the mill, and let fall 
upon the wedge, till it has driven it down as far as it 
will go. This caufes a molt immenfe preflure upon the 
feed contained in the bags, and forces out the oil at every 
blow of the ftamper : for it is a curious fadt, that the fame 
preflure, gradually produced, will not exprefs any fenfi- 
bie quantity of oil; it muft, therefore, be done by a Hid¬ 
den and violent acceffion of force ; accordingly, at every 
Itroke upon the wedge, the oil exudes in confiderable 
quantity from the feed until the whole is exprefled, leav¬ 
ing a cake of feed as hard as a piece of board. Thefe oil¬ 
cakes are very good food for cattle. 
The annexed Plate exhibits the machinery of Smeaton's 
Oil-mill, where fig. i. is the plan, and fig. 2. the elevation. 
A, in both figures, is the water-wheel actuating the whole 
mill; it is underfhot; that is, the water paffes underit, and 
turns the wheel byits momentum ftrikingupon the floats. 
I L. 
It works very clofe in a pit formed between the two walls 
c, C, the latter being theoutfide wall of the mill-houfe : it 
is framed on a ftrong ociagonal fhaft B B, turning on two 
gudgeons fixed in its ends : D is the pit-wheel, or great 
cog-wheel, fixed upon the main fhaft, within the houfe: 
it has teeth fixed in its rim, parallel to its axis, in the 
manner of a crown-wheel. There are eighty of thefe 
teeth, by which it turns a fmailer wheel, E,- of thirty- 
feven teeth, fixed on an horizontal fhaft, F, called the 
tumbling-fhaft, extending nearly acrofs the houfe. It 
gives motion to the (tampers, V, W, of the prefs, and 
alfo the rolling-ftones or runners, I, K, which bruife the 
feed. The latter is done by means of a wheel, G, with 
thirty-five teeth, fixed on the end of it, for the purpofe 
of turning a large wheel, H, of feventy-two teeth, which, 
is fixed upon a vertical fhaft, nn, which gives motion to 
the running-ftones, I, K. Thefe are two circular ftones, 
fitted upon an iron axle, the en'ds of which are fhown at 
a a. This axle paffes through the vertical fhaft 7 >i, and 
alfo through the runners : thus they have two motions, a 
rotation round their own axis, by which they are carried 
round upon the net her or horizontal mil l-ftone,L, on which 
they roil. The centre-holes in thefe running mill-ftones 
are made a little wideband the hole in the fhaft m, which 
carries the middle of the iron axis, is made oval up and 
down. This great freedom of motion is neceflary for 
the runner mill-ftones, becaufe frequently more or.lefs 
of the grain is below them at a time, and they muft 
therefore be at liberty to get over it without /training the 
fhaft. 
The lower mill-ftone, L, is fupported on mafonry, and 
furrounded by a border or ledge of wood, o o, which 
prevents the feed being fcattered : the two ftones, I, K, 
as the figure fliows, are piaced at different dilfances from 
the central axis m, fo that they run in different paths, and 
thus bruife the feed more effectually than if they followed 
each other in the fame circuit: nn, are the ends of two 
wooden rails, projecting from the fhaft m, and at their 
ends fupporting two upright pieces of wood, which, at 
their inferior extremities, carry fweeps or rakes: thefe 
drag round upon the furface of the lower ftone, and turn 
the feed about, to receive the greateft poffible action from 
the ftones, I, K, rolling over it. There are two of thefe 
fweeps on the oppofite fides of the ftones, one called the 
outer rake, and the other the inner rake. The outer 
rake collects the grain under the runner from the furface 
of the border o. In this manner the grain is turned over 
and over, and crufhed in every direction. The inner 
rake lays the grain in a Hope or ridge, over which the 
runners pafs, and crufh it: then the fecond rake lifts it 
again into a ridge, to receive the adtion of the next ftone, 
fo that every fide of the grain is prefented to the runner 
mill-ftone, and receives its adtion, while the reft of the 
lower ftone is fwept by them fo clean, that not a fingie 
grain is left on any part of it. Theouterrake is alfo fur- 
nifhed with a rag of cloth, which rubs againft the border 
of a hoop which furrounds the nether mill-ftone, fo as 
to drag out the few grains which might otherwife remain 
in the corners. There is alfo anotherfweep, making part of 
the inner rake, which is occafionally letdown forfweeping 
off ail the feed, when it has not been fufficiently bruifed. 
The preflure and adtion of thefe rakes are adjufted by 
means of wooden fprings, which cannot be eafily and dil- 
tindtiy reprefented by any figure. The oblique pofition 
of the rakes (the outerpoint goingforemoft) caufes the’bt 
to fhove the grain inwards, or towards the centre, and 
at the fame time to turn it over fomewhat in the fame 
manner as the mould-board of a plough flioves the earth 
to one fide, and partly turns it over. Some mills have ' 
but one fweeper; and indeed there is great variety in 
the form and conftrudtion of this part of the machinery. 
The great pit-wheel, D, turns a fmall cog-wheel, e, fig. 
1. on whole fpindle is fixed one of the two iron-rollers, 
/> g> which are ufed for bruifing the feed, as in the firft 
* ' operation. 
