20 SEC. 2.- -GEOMETRY. 



point in nature which may be chosen as the point of view. The sights of the 

 instrument are then directed to any point in nature which it may be desirable 

 to introduce into the picture. This may be done by the lateral and vertical 

 movements of the horizontal disc and plate to which the sights are attached ; 

 the divisions and subdivisions indicated by the zero mark on the horizontal 

 disc, and index on the side arc, are then read off and transferred to the picture 

 by means of the graduated scale and offset, the zero of the scale being placed 

 to the point of view and the bevelled edge to the horizontal line, the offset is 

 then moved along the edge of the scale till the bevelled edge corresponds with 

 the division or subdivision read off from the circular disc, when a slight dot 

 with the point of a pencil at the division or subdivision on the offset corre- 

 sponding to the reading on the side vertical arc will indicate the point which is 

 the perspective representation of the point in nature. The scale can be in- 

 creased, if desired, by multiplying the number of divisions or subdivisions read 

 off on the graduated arcs by 2, 3, 4, or any other common multiplier. The 

 offset also may be dispensed with by placing the edge of the scale first to the 

 horizontal line, for the first reading making a slight mark on the line with a 

 pencil at the number indicated, and then perpendicular to it for the second 

 reading, this maybe the most convenient mode of laying down the points 

 when the sketching book is held in the hand. The stand of the instrument 

 folds up, and the apparatus when packed is very light and portable, consisting 

 of camp stool, drawing book, instrument and stand. 



443a. Plate Glass Sector, designed for the purpose of 

 plotting angles on plans or charts where it is necessary to see the 

 work under the sector, and the divisions being . on the side next 

 the paper no variation in pricking off can take place, 



Thos. F. Chappe, M. Inst. C.E. 



Boxwood Beam Compasses. 



Bock and Handrick, Dresden. 



244. Scales, of boxwood, showing the equivalents of English 

 and Foreign measures of length. Aston fy Mander. 



The bevel edged set square slide is used to show the divisions coinciding ; 

 and the equivalent values of English and Foreign measures of length may thus 

 be readily obtained. 



245. Plotting Scales. Ivory. Two specimens, to show fine 

 and accurate dividing. Aston fy Mander. 



No. 1 shows two chains to the inch, represented by 200 divisions to the 

 inch. 



No. 2 shows one chain to the inch, represented by 100 divisions to the 

 inch. 



II. INSTRUMENTS FOR TRACING SPECIAL CURVES. 



7O. Conograph. An instrument by which the various conic 

 sections may be drawn. . 



a. Ellipso-Pnrabolograph. 



b. Hyperbolograph. 



Dr. Lawrence Zmurko, Lemberg. 



