DESCRIPTION OF PLATES. 



PLATE VIII. 



Fig. 103. The heights of the houses, windows, doors, and figures are determined 

 by lines directed to the centre of the picture ; the true height being measured on the 

 lines AB, CD, where the objects are supposed to touch the plane of projection. 

 The distance E F, and all other parts of lines perpendicular to the picture, are 

 measured by laying off the lengths of the originals, as G H, on the line AC, and 

 drawing I E G, I F H, from I, the point of distance ; which, in most cases, will be 

 more remote from the centre of the picture than it is here made. The line KL, 

 and others parallel to A C, may be measured by the assistance of any point M in the 

 horizontal line, the distances, NO, OP, being laid off on AC, or simply by reduc- 

 ing the scale in the proportion of M P to M L. P. 89. 



Fig. 104. A circle thrown into perspective, by means of the circumscribed square, 

 the points of contact being found by bisecting the sides. P. 89. 



Fig. 105. Two perspective delineations, and two orthographical projections of a 

 cube, in different positions. For the orthographical projection, the ground plan 

 being A B C D, the image of any point A, B, may be found by drawing A E, B F, 

 perpendicular to the ground line ; E G, F H, parallel to the line assumed for the di- 

 rection of the centre of the picture, and A G, B H parallel to the line of direction of 

 the point of distance ; the intersections G and H will then be the points correspond- 

 ing to A and B. P. 90. 



Fig. 106. A is the orthographical projection of a sphere, with some of its circles; 

 B the stereographical projection of the same circles. P. 90. 



Fig. 107. A balance made by Fidler for the Royal Institution, nearly resembling 

 those of Ramsden and Troughton. The middle column A is raised at pleasure by 

 the cock B, and carries the round ends of the axis in the forks at its upper part, in 

 order to remove the pressure on the sharp edges of the axis within the forks. The 

 scales are occasionally supported by the pillars C and D, which are elevated or de- 

 pressed by turning the handle E. The screw F serves for raising or lowering a weight 

 within the conical beam, by means of which the place of the centre of gravity is 

 regulated. The extent of the vibrations is measured on the graduated arc G. 

 P. 97. 



Fig. 108. A balance for the illustration of different kinds of equDibrium. When 

 the scales are hung on the middle pins, A, B, which are in the same horizontal line 

 with the support of the beam, the equilibrium is neutral, the weights acting as if the 

 centre of gravity coincided with the point of suspension. If the scales be hung on 

 the lowest pins C, D, the centre of gravity will be nearly in the line C D, and its 

 path the curve E, which has its concavity upwards ; but if the scales are hung on the 

 pins F, G, the path of the centre of gravity will be convex upwards, and the beam 

 will overset. In reality the true paths of the centre of gravity would be nearly in 

 the curves H and I, situated between the weights in the scales : but these are similar 

 to the other curves. P. 97. 



Fig. 109. When the equilibrium of a balance is tottering, the lower weight acts 

 with the greatest advantage : thus the effect of the weight A is reduced in the pro- 

 portion of B C to D C, by the obliquity of the arm C A, while the weight E acts on 

 the whole length of its arm C F. P. 97. 



Fig. 110. If A B C be a semicircle, and B D represent a given weight, and A D 

 its counterpoise in one of the scales of an unequal balance, D C will be the counterpoise 

 in the other scale. It is obvious that A C is more than twice as great as B D. 

 P. 97. 



