CYCLOGRAPH, or ARCOGRAPII, is an instrument for drawing arcs of circles without centres, used in architectural and engineering drawing when the centres aretoo distant to be conveniently accessible. Bricklayers and ma sons when they wish to strike an arc upon the face of a wall, have recourse to a very simple but perfect mode of accomplishing the object, by driving a nail into the wall at each extre mity of the intended are, and then nailing two straight laths or rods together at such an angle, that while their external sides or edges are in contact with the nails driven in the wall, their apex or meeting point shall touch the crown of the required arc or arch. When se cured to each other at the required angle, the laths are so moved that, while they remain in contact with the nails, the apex may traverse the whole distance from one nail to another, in doing which it will describe the required curve, which may be marked on the wall by a piece of chalk carried round with it. The same plan may be adopted in drawing on paper, substituting pins for the nails, and a piece of stout cardboard, cut to the required angle, for the laths. Mr. Botch's Arcoyraph,
Mr. Alderson's Curvilinead, Mr. Warcup's Cur vograph, and Mr. Peter Nicholson's Cenirali nead, all described in the TransactionP of the Society of Arts, are instruments for pro ducing somewhat similar results. The centro linead may be compared to a T-rule, in which the transom consists of two pieces adjustible to any required angle with each other, and the centre of which, answering to the apex, of the cyclographs above described, is precisely on a line with the fiducial or drawing edge of the stem or long limb of the rule. The instru ment being once adjusted to the required an gles, and having its angular transom laid against two fixed pins, just like the angle of a cyclograph, any number of converging lines may be drawn by it as readily as parallel lines are drawn by a common T-rule.