3 Deseription of a Mould Board,&t. TJuly, tended only to raise: be clod of eartb; but as it is also to be turned over, the left half of the upper wedge has been pre- Served, in order to continue o1n the Same Side the Slope which is 10 be made upon the right halt of the lower wedge.

We will now proceed to form ihis Slope, tbe object for which 80 much precaution has been taken in tracing the lines round the block,'previous to removing thie pyramid, fg. 4 Attention must be paid not to conjound tbese lines, now they are Separated by the vacancy lett by removal of the pyramid, Üg=5. Enter the Saw upon ihe two points of the frst line, Situated at the places where this was interrupted, and which are the two points of interzection with tbe exterior diagonals; ok mk, continuing the cut of the frst line until 11 arnives at one, part of the central diagonal k(, and on the other at the right inferior edge 04h, of the block, Bg 5; the lower end of the saw will go out by Some point Situated upon the Superior mark, in a night line with the corresponding points vf the edge, and the central diagopal. Continue the Same upon all the points formed by he interSections of the exterior diagonals, and the lines traced round the block, always taking the cen- wal diagonal and the edge 0 h lor the term and the traces as guides; it will happen ibat when you have made many ot these cuts with the Saw, the end ot the instrument which went out hitberto by the Superior face of ihe block, will go aut by the face Situate on its left, and all these different cuts of the 5aw will have marked 80 wany right lines, w bich in leaving the inferior end 0 Ah of the block, will eut or intersect tbe cen- tral diagonal. Then with a couvenient tool, take away the Parts 3awed off, observing only to leave the marks of the Saw

(e] vieible, and the face of the eartl board will be fnished.*

* The figures 9 and 109 which are here added to accompany the memoirs of Mr. Jefferſon, have been drawn in perlpc&ive, by Citizen Valencienne, aſliſt- ant naturaliſtof the Muſcum, and will ſerve to eXplain clearer than by writing the reſult of the operation.

Suppoſe that the ſaw cuts the Jines mk ok Fig 9, at the points x and t; taken on the traces xf and ts, fitnated in the ſame plan parrallel to barc, and of which the prolongation upon the triangles mk Land ok) are the lines xz and tz, the ſaw ſhould immediately penetrate the block, continuing In the plan in which it ads till its edge is arrived at the point S, and at the fame time touching the point z of the central diagonal k 1. rhe fame edge of the[aw will go out by ſome point y of the face m kb, ſo that' the three points s zy will be on the lame diredtion. Or if the ſame operation is re pcated on different parts of the lines mk, ok b, from'k up t0 aceitain height; the points of the face m k b by which the law will go out, will form a curve k yn. This height being paſed, the ſaw always directed in ſuch a manner, tnat at the end of its motion, it touches at-the lame time the edge oh and the central diagonal k 1, will go out by other points ſituated on t1€ lower face a bm!, and the end of thele paints will form a iecund Curve

n 1, which will meet the firſt at the point n»« a