DESCRIPTION OF PLATES. 



PLATE VII. 



Fig. 91. A Sector. The scale of equal parts is marked L. As A B is to A C, 

 so is B D to C E ; and if any line B D be placed with its extremities in the third 

 division of the scale on each leg, the distance C E between the seventh divisions will 

 contain 7 equal parts, of which B D contains 3 ; and the same is true of any other 

 numbers. P. 80. 



Fig. 92. A vernier, indicating 38$, of the divisions of its scale. P. 81. 



Fig. 93. A sliding rule. The slider being drawn out, so that the division marked 

 1 is opposite to 3 on the rule ; all the other figures on the rule are triple of those 

 which stand opposite to them. P. 82. 



Fig. 94. A circular logarithmic instrument. The inner circle slides within the 

 outer, and as it is represented in the figure, each number stands opposite to another 

 which is twice as great. P. 82. 



Fig. 95. A steel chain, made by Ramsden. A, the screw for bringing the mark 

 B precisely to the point required ; C, a joint between the adjoining links ; D, a 

 cross joint at every tenth link ; E, a pulley and weight for stretching the chain. 

 P. 86. 



Fig. 96. A micrometrical scale made by Troughton. The compound microscopes 

 A and B are fixed nearly at the required distance on the scale C ; A is then made to 

 point exactly to a division of the standard scale D by means of the screw E, and B 

 to another division, at the required distance, by means of the screw F, the fractional 

 parts being added by the turns of the crew G. The scale D is then removed, and 

 the object to be compared with it is put in its place. P. 86. 



Fig. 97. A diagonal scale. The line A B contains 274 parts, of which the units 

 of the scale contain 100. P. 86. 



Fig. 98. The statuary's compass, seen sideways. The pin AB is forced.down, 

 till it is stopped by the moveable stud C ; the screw D fixes it in its angular posi- 

 tion. It is also capable of motion round the axis E F, which is fixed by the screw 

 G. P. 87. 



Fig. 99. An instrument for making drawings in perspective ; the perforated 

 sight may be drawn out to any required distance. The dotted lines show how a 

 second frame may be applied instead of the sight, so as to answer the same purpose. 

 P. 88. 



Fig. 100. Illustration of the principles of perspective. A being the place of 

 the eye, and B C the plane of projection, if A D be parallel to E F, G H, and I K, 

 D will be their vanishing point, and E D, G D, and I D, their whole images : A L 

 being parallel to E M and I N, L will be their vanishing point, and EL, I L, their 

 whole images : and A O being parallel to P Q, O will be its vanishing point. 

 P. 89. 



Fig. 101. A being the centre of the picture, A B the horizontal vanishing line, 

 A C the vertical line, and D the point of distance, if a ground plan E F G H of any 

 figure on the horizontal plane be placed in its true position with respect to I K, the 

 bottom of the picture, the vanishing points of all its lines will be found by drawing 

 DL, DM, D N, and D O, parallel to those lines respectively; and the whole 

 images of the lines will be P L, Q M, K N, and I O, determining, by their inter- 

 sections, the figure R S T U, which will be the projection of E F G H. The plan 

 may also be drawn, in an inverted position, below the line I K, and the point of 

 distance taken above A instead of below it. P. 89. 



Fig. 102. A B being the whole image of the line represented by A C as a ground 

 plan, and D the point of distance, we may find E, the image of the point C, by 

 drawing C D ; or we may make B F=B D and A G=A C, F G will then also cut 

 A B in the point E. P. 89. 





