r64 



PLATE VII. 



Fig. 91. A sector. The scale of equal parts is 

 marked L. As A B is to AC, so is B D to C E ; and 

 if any line R D be placed with its extremities in the 

 third division of the scale on each leg, tfie distance 

 C E between the seventh divisions will contain 7 equal 

 parts, of which B D contains 3 ; and the sam« is true 

 of any other numbers. P. 104. 



Fig. 92. A vernier, indicating 38ot of the divisions 

 of its scale. P. 105. 



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 

 tliose which stand opposite to them. P. 107. 



Fij^. 9t. A circular logarithmic instrument. The 

 inner circle slides within the outer, and as it is represent- 

 ed in the figure, each number stands opposite to an- 

 other which is twice as great. P. 107. 



Fig. 95. A steel chain, made by llarasden. A,«the 

 •crew 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. 11?. 



Fig. 9(5. A micronietrical scale made by Troughton. 

 The compound microscopes A and B are fixed nearly 

 »t the required distance on the scale C : A is the;i 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 

 fractiona parts being added by the turns of the screw 

 G. The scale D is then removed, and the object to 

 be compared with it is put in its place. P. 112. 



Fig. 97. A diagonal scale. The line A B contains 

 S74 parts, of which the units of the scale contain 100. 

 P. 112. 



Fig. 98. The statuary's compass, seen sideways. 

 The pin .IB is forced down, till it is stopped by th« 



moveable stud C ; the screw D fixes it in its angular 

 position. It is also capable of motion round the axis 

 E F, which is fixed by the screw G. P. 113. 



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 

 cecond frame may be applied instead of the sight, so 

 as to answer the same purpose. P. 115. 



Fig. 100. Illustration of the princij;les of pcrspec 

 vc. A being the place of the eye, and BC the plane 

 of proj ection, if A D be parallel to E F, G 11, and I K, D 

 will be their vanishing point, and E D, G 1), and I D, 

 thtir whole images : AL being parallel to EM and IN, 

 L will be their vanishing point, and EL,IL, their 

 whole images : and A O being parallel to P Q, O will 

 be its vanishing point. P. 115. 



Fig. 101. A being the centre of the picture, A D the 

 horizontal vanishing line, AC the vertical line, and D the 

 point of distance, if a ground plan EFGHofany figure 

 on the horizontal plane be placed in its true position with 

 respect to I K, the bottom of the picture, the vanish- 

 ing points of all its lines will be found by drawing DL, 

 D M, D N, and D O, parallel to those lines respect- 

 ively ; and the whole images of the lines will be PL, 

 QM, KN, and 10, determining, by their intersec- 

 tions, the figure IISTU, which will be the projec- 

 tion ofEFGH. 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. 115. 



Fig. 102. A B being the whole image of the line re- 

 presented 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 CD; or we may make BF^rRD and 

 AG=AC, FG will then also cut AB in the point E. 

 P. 116. 



