viii DESCRIPTION OF PLATES. 



PLATE VI. 



Fig. 77. The proportions of the diameters of the different parts of the double 

 pullies being 3 to 2, 5 to 2, and 8 to 2, the middle weight may be observed to rise 

 the most rapidly. P. 67. 



Fig. 78. A wheel supposed to be capable of producing a perpetual motion ; the 

 descending balls, acting at a greater distance from the centre, but being fewer 

 in number, than the ascending. In the model, the balls may be kept in their 

 places by a plate of glass covering the wheel. P. 70. 



Fig. 79. A, the inclination of cross lines generally most convenient for pro- 

 ducing the effect of a tint, in drawing. B shows the effect of lines crossing each 

 other perpendicularly, and C that of lines crossing too obliquely. Where the sur- 

 face to be shaded is large, the separate lines or hatches should begin and end with 

 a point, in order that the junction of the different portions may escape observation. 

 P. 72. 



Fig. 80. Dr. Hooke's telegraph, in which the characters are arranged behind a 

 screen, and drawn out as they are required. P. 77. 



Fig.' 81. Dr. Hooke's alphabet, with some other arbitrary characters for his tele- 

 graph. P. 77. 



Fig. 82. A beam compass, with a scale. P. 78. 



Fig. 83. ..85. Instruments for drawing arcs of large circles. P. 78. 



Fig. 86. A pair of triangular compasses. P. 78. 



Fig. 87. Marquois's scales, for drawing parallel lines. P. 79. 



Fig. 88. A pen for ruling musical lines. P. 79. 



Fig. 89. A pantograph. A being the centre of motion, B the tracing point, 

 and C the describing point. A B is always to AC as A D to A E, and the copy F is 

 similar to the original G. P. 79. 



Fig. 90. A pair of proportional compasses. P. 79. 



