It does not fall within the province of this work to trace the ciramestances which cell for the varied modes of ploughing. They are numerous, depending both on the erop and on the coil. For some purposes the clod requires to be turned right over ; for others, to be turned to a certain angle. Very little attention was formerly paid to the straightness of the furrows ; but it. is evident that every deviation from parallelism muses a defect in the contact of the slices, and a loss of force by the obliquity of the draught ilenee equal and straight farrows are a sign of good ploughing. One of the most useful operations in ploughing land is to cross the former farrows, by which means the whole soil is much more completely stirred ; and if any part has been left solid without being moved by the plotter-share, which is called a Balk, it is now necessarily moved.
As a means to enable the farmer to wave bands and expedite the tillage of the land, ploughs have been contrived which make two or more farrows at once. Wheat they are well constiteoted, they am veryuselld on light soils. If it is net required to go deep, and two horses can draw a double plough, thete is a decided saving of power; but if it requires four horses nothing is gained. The double ploughs are therefore not math use.
Deep ploughing is generally imknowlesiged to amoral with the best husbandry, where the subsoil is ek. naturally, or has been artificially drained; but some inconvenience may arise fnom bringing a barren subsoil to the surface, in trenth-plowehing by two ploughs follow* each other in the same farrow. It has there fore been Etigge6ta to take oil the turn-furrow from the plough which follows the first, so as to stir the subsoil without bringing it to the surface- This idea hat been improved *pen by contrasting a &abseil-plough of sink strength, which trill go eel's deep Into the grouttd and stir the subsoil * foot or Mere below the bottom of the usual farrete.
The instruments which have bean invented to save the time and labour Modred by re peated ploughings are Very numerous, New ones are daily invented, end some are sup ported by wheels, which render them both lighter and more convenient. One of the most important of the instranhente is the Cultivator, or Scarifier. LCPLTIVITOR.1 It would be in vein to teatime...rate the vuri ens ploughs which hare been itivented and which are exhibited at the annual cattle and agricultural shows. The Essex plough, the Gold hanger plough, the Subsoil plough, the Surface plough ; as well as those which are designated by the names of their makers, Ransom°, Croaskill, Wedlake, Garmtt, &se...— all have their respective points of excellence. There has recently been introduced a hill-side plough. Several forms of steam pkety/s, also, have been introduced t in which steam-power takes the place of horse power. A steam plough has been recently patented by Messrs. Calloway and Pttrkis of Putney. There are two plottghs attached to en endless chain which passes over two Wheels ; and these wheels are meted whenever the 1os:emotive movement of the steam-engine tabs place. The ploughing is effected transversely to the movement of the machine; and this mont is only just rapid enough to enable the plough to act upon new portions of ground when necessary.
PLUMBA'GO—Gmphite, Mere Lead. This substance occurs broth erystallised and massive; but Mostly in irregular nodules. It is found in various parts of the world. That of Bor in Cumberland is of the best quality for what are called Wick-keel „ewits; while the commoner starts are used for making knelting-pots, for diminishing the friction of machinery, and for protecting iron front resting. That which is Maported from the East Indies is remarkably soft. Plumbago conducts electricity, is inftmible, and very difficult of combustion, It Was at one time supposed that plezabago ants a earburet of iron, but, in the opinion of Bereelius, it is a peculiar form of earboa, and that the substances which it contains are in A state of mixture merely and not of chemical combination.
Pinmbago is found usually in primary or transition rocks. At Borrowdele it occurs in nests in a greenstote rock, which constitutes a bed in clay-slate. In Inverness-shire it wears in gneiss; at Areedal in Norway, in quarts rock ; and in the United states, in felspar and mica-slate, but always in primary rocks. The Borrowdele mine is interesting, and tAl Deady was eke materiel fox Ezeriebt preemie It as shout evx :gibes freak Keswick. It is about hied way no a monesain two tlimmand feet hie% soeseeten difficult ei Amen. of that sew' les pne awed are pare pinning); while seisms have pertieies of ire% ore adhering m them. Fran tae moat supply of the inseam, and Coe high price which it oemeamas in the en:eget, the wining epee:Sow Are coodumed Isiah tenth yereme 'nee and even secrecy. 14 tile list eeaue7 persona contrived to red) the nine of meth eeteaare with 'eparity ; and teiiu la to a very stringent set of razes fsweeasz. The mine is now nearly exhausted.
The netnefactore of idea-lead peeeile frost this wabstanm is noticed under Pte:, The can of which the eases are formed is sawed into long *asks, and subsequently itue illtiarr rasa ; grooves are tot oat by muse of a 11y-wheel, of Each a size as to eseeive the ids* -lead. The pieces of mitered are en into thin *Ube, and than nee rods i tined rode are inserted in the grooves, and the two balsas of the ease are glued together.
A material for panels is very swath visa, eepeenEy in Francs, mac pinaliego mimed up with say fine clay. Any required degree of headset can he Liven to the compeenion by varying the proportion of the ingediente. Weir "purified black-lead pensele,' scrod ie London, hen thirteen deferent der rise of hariniess. Mr. Ensciseclin and other pagans also manefacture pezunis is ehieit the irei mess and the Us:ham eery in any denessi digger.
When a hem body sus pended by a Parade string is at net, the line pass* through the pint of seeepeneion and the centre of eravity of the weight is in the direeden in which gravity arm The Horizon is a plant perpendieehr to this Ern, and the ZewitA is that point of the heavens marked eat by the continua-Ices of the line upwards. If the string be perfectly flexible, it will with the theoretic) line shove de seribecL The plirmbEne is used In the arts. In levelling end in astronomical inAnneects, it has in most eases been superseded by the spirit level or observation by selection.