ADDITIONS AND CORRECTIONS. 



p. 40. L. 5 from the bottom; for "therefore," 

 read, afterwards. 



P. ri.- L. 5 from the bottom; for " IV," read, V. 

 P. 72. L. 2, for " IV," read, V. 

 P. 87. After 1. 4, insert. 



When an insulated body revolves round an axis in 

 any direction, the state of revolution cannot be per- 

 manent, unless the axis be so situated, that the cen- 

 trifugal forces on each side of it balance each other. 

 It is obvious that this must happen in a homogeneous 

 sphere, whatever may be the situation of the axis ; 

 and it has been demonstrated, that when the body is of 

 an irregular form, there are at least three rlifterent 

 axes, situated at right angles to each other, round 

 which the body may revolve in an equilibrium either 

 stable or tottering. It may also be shown that if a 

 body, revolving round any axis, receive at the same 

 time an impulse which would cause it to revolve round 

 a second axis in another direction, the two revcrfutions 

 will be combined, and will form a single revolution 

 round a third axis, in an intermediate position, which 

 will remain at rest until it be displaced by some new 

 force, provided that it be one of the axes of perma- 

 nent revolution: so that no body can revolve round a 

 moveable axis without a continual disturbing force. 

 And when an irregular body begins to move on an 

 axis incapable of equilibrium, its revolution will be 

 gradually altered, so as to approach continually to a 

 revolution round one of the natural axes; bnt it will 

 never pass beyond the state of equilibrium, as in many 

 other cases of deviation from such a state ; since the 

 momentum, produced by the excess of centrifugal 

 force in one part of the revolution, is destroyed irf 

 another. For a similar reason, if a stick be thrown in a, 

 horizontal position, with a rotatory motion, it will fall 

 in the same position much more certainly than if it 

 were thrown without any rotation; for any small dis- 

 turbing force, which might be sufficient to turn it into 

 a vertical position during the course of its path, will 

 only produce, when combined with the rotatory mo- 

 tion, a slight change of the direction of the rotation, 

 which will confine the deviation of the stick from a 

 horizontal position within narrow limits. 



P. 138. L. 9, after " concerned," insert, it has in- 

 deed been asserted that the specific gravity of elastic 

 gum is even diminished by tension, so that the actual 

 distances of the particles cannot, in this cas«, be sup- 

 posed to be materially increased. 



P. 146. L. 3, after " 124," insert, 125. 



L. 8 from the bottom, for « IX," read, X. 

 P. 169. L. 7, for "XIV, read, XIII. 

 P. 176. L. 19, for " the circle," read, a second 

 circle. 



P. 196. L. 5 . . 2, from the bottom, for " If the fric- 

 tion. . to obviate this," read, Since friction is always 

 increased by an increase of pressure, the effect of any 

 addition to the sustaining force must tend, in some de- 

 gree, to retard the vibrations, even if the friction be 

 somewhat less increased than the force propelling the 

 balance. In order to obviate this retardation. 



P. 238. L. 5 from the bottom, after " arches," in- 

 sert, since they must have left too small a space for 

 the passage of the water. If, however, we may be- 

 lieve Herodotus, whom Mr. King has quoted, this was 

 in reality a kind of drawbridge. According to thi» 

 author, it was built by Nitocris, the immediate Suc- 

 cessor of Serairamis: the stones were united by iron 

 and lead, and beams were laid across them, which 

 viere removed at night, in order to prevent the mutual 

 depredations of the inhabitants of difterent parts of 

 the city. 



P. 261. L. 19, for "XX," read, XIX. 

 P. 267. L. 18, for " heel," read, pitch. 

 P. 273. L. 3 from the bottom, omit " logarithm of." 

 L. 2 from the bottom, for " numbers," 

 read, corresponding logarithms. 

 P. 292. L. 9, for " de," read, du. 

 P. 420. L. *, for "more," read, most. 

 P. 424, after line 5, insert. 



Dr. Wollaston has/ very ingeniously applied the- 

 effect of the reflection of two plane surfaces, inclined 

 to each other, to the construction of an instrument for 

 drawing, which he calls a camera lucida. He usually^ 

 employs the internal reflection of a prism of glass, of 

 which the four surfaces are ground so as to form pro- 

 per angles with each other. The image formed by-, 

 the first surface is inverted, and the second reflection 

 restores it to its original position, but places it in a 

 direction which is at right angles with the direction of 

 the object ; so that when we look down through the 

 prism on a sheet of paper, we see a perfect picture of 

 the objects immediately before us, while at the same 

 time the aperture, through which we look, is only partly 

 occupied by the edge of the prism, the remaining part 

 being left open, or simply covered with a lens, for the 

 adraissjon of the direct rays of light, by which we may 



