Description of a Ploiigk-ear. 83 



traces will serve to show how the wedge which is at the 

 right anglu rises gradually on the direct or loAvcr face of the 

 wedge, the inclination of which is preserved in the central 

 diagonal. One may easily conceive and render sensible tlic 

 iiianner in v:\nz\\ the sod is raised on tlie ear, which we 

 have described, by tracing out on the ground a parallelogram 

 two feet long and nine inches broad, as abed (fig 6) : then 

 placing in the point /- the end of a stick 27^ inchefm length, 

 and raising the other end 12 inches above the point c: \\X\c. 

 line de, equal to 4^- inches, represents the quantity which 

 the height of the ear exceeds the perpendicular). When 

 this is done, take another slick 12 inches in Lnith, and, 

 placing it on ah, make it move backwards, and parallel to 

 Itself h-om aZ; to cd, taking care to keep one of its ends al- 

 ways on the line ad; while the other end njoves along the 

 &tick be, which here represents the central diagonal. The 

 motion of this stick of 12 inches in length wfll be that of 

 our ascending wedge, and will show how each transverse 

 hne of the sod is carried from its first horizontal position 

 nntil It be raised to a height which exceeds the perpendicular 

 so much as to make it fall inverted by its own weight. 



But to return to our operation : — it remains to construct 

 the lower part of the ear. Invert the block and make the 

 saw enter at the points where the line al (fio-. (j.) meets 

 wiih the traces, and continue your stroke along^'these traces 

 until both ends of the saw approach within arriach, or any 

 other convenient thickness, of the opposite face of the ear. 

 When the cuts are iini^hed, remove, as before, the sawn 

 pieces, and the ear will be finished *. 



It 



the same straight Wne. Vmi if this operation bo repeated in diiFerent plares 

 of tlie hncs vik, o kb frou. / to ;i ccrcuin height, the points of tin- fare m kb, 

 at which the saw com.-s out, will form a curve kyn. Kevond this height 

 ■the fuw, always directed la .such a niunncr that at the cr.d of its mciion it 

 ehall touch at the s;in.e lime the ed^e ok and the central diagonal k I, will 

 come out at oliicr points situated im; the posterior f^ce almt, and tlie scries 

 ■ol ttrcse points will form a second curve n I, v.hicli will nice: the first in the 

 point u. The.c tv/o curves being traced out, let us suppose straight lines 

 ■drawn to the places where the saw stopfjcd each time 'thiU it touched the 

 diagonal/./, and of which one, as a!re;-'.dv said, passes through the points 

 «,i,2/; and let us conceive a surface touching all these straight lines, and 

 wiio-c liinirs, oil tiu2 one hand, shall be the curves ///?;, nl, and on the other 

 the edge c,/i, ;his surface, which must be uncoven-d bv sections made with 

 a proper instruu^ent, will form or.e of the faces of the ear. The latter is re- 

 prt^seuttd fig. 10, a;.d the face ia question is that which appears before, zs.d 

 ■which IS indicated by inlur. It will be remarked that the angle situated to- 

 wards /; (fig. [>) on the part k(.dich of the block has also been cu; off bv a 

 section made Jrom d to r, agreeably to what will be said hereafter.— A'«{c' uf 

 tut i'rcrih Edttur. ' 



• Wc •}*!] here ,-tdd to tliii descrlptiori an iUustrstiwC siaiilar to that given 



