450 



PHYSICAL SCIENCES. 



is the same as if it were two forces, one of which acting in the direction 

 b m, diminishes the force of gravity, and another which, acting in the direc- 



Fig. 30. 



tion m t, tangent to the sur- 

 face at m, urges the particles 

 towards Q, and tends to swell 

 out the earth at the equator. 



NOTE 119, p. 45. Homo- 

 geneous mass. A quantity of 

 matter, everywhere of the 

 same density. 



NOTE 120, p. 45. Ellip- 

 soid of revolution. A solid 

 formed by the revolution of 

 an ellipse about its axis. If 

 the ellipse revolve about its 

 minor axis Q D, fig. 6, the 

 ellipsoid will be oblate, or 

 flattened at the poles like an 



orange. If the revolution be about the greater axis A P, the ellipsoid 



will be prolate, like an egg. 



NOTE 121, p. 45. Concentric elliptical strata. Strata, or layers, 

 having an elliptical form and the same centre. 



NOTE 122, p. 46. On the whole, fyc. The line N Q Sq, fig. 1, repre- 

 sents the ellipse in question, its major axis being Q q, its minor axis N S. 



NOTE 123, p. 46. Increase in the length of the radii, $c. The radii 

 gradually increase from the polar radius C N, fig. 30, which is least, to the 

 equatorial radius C Q, which is greatest. There is also an increase in the 

 lengths of the arcs corresponding to the same number of degrees from the 

 equator to the poles ; for, the angle N C r being equal to q C d, the ellip- 

 tical arc N r is less than q d. 



NOTE 124, p. 46. Cosine of latitude. The angles mCa, mCb, 

 fig. 4, being the latitudes of the points a, 6, &c., the cosines are C q, 

 C r, &c. 



NOTE 125, p. 47. An arc of the meridian. Let NQ Sq, fig. 30, be 

 the meridian, and m n the arc to be measured. Then, if Z'm, Zn, be 

 verticals, or lines perpendicular to the surface of the earth, at the extremi- 

 ties of the arc m n they will meet in p. Q a n, Q 6 m, are the latitudes of 

 the points m and n, and their difference is the angle mp n. Since the lati- 

 tudes are equal to the height of the pole of the equinoctial above the 

 horizon of the places m and n, the angle mp n may be found by observa- 

 tion. When the distance m n is measured in feet or fathoms, and divided 

 by the number of degrees and parts of a degree contained in the angle 

 mp n, the length of an arc of one degree is obtained. 



NOTE 126, p. 47. A series of triangles. Let MM', fig. 31, be the 

 meridian of any place. A line A B is measured with rods, on level ground, 

 of any number of fathoms, C being some point seen from both ends of it. 

 As two of the angles of the triangle ABC can be measured, the lengths of 

 the sides A C, B C, can be computed ; and if the angle m A B, which the 



