=

cubic feet, the solidity of the sector ACDE (Prob. XXI.); 26.462 X 0.7854 X 15 2946.99 cubic feet, the solidity of the cone AB-C (Prob. XV.); 4188.8 -2946.99 1241.81 cubic feet, the solidity of the segment ABE required.

=

2. Required the solidity of a segment, whose altitude is 57 inches, the diameter of the sphere being 153 inches.

3. What is the solidity of a spherical segment, whose altitude is 13 feet, and the diameter of the sphere 33 feet 6 inches?

4. Required the solidity of the segments of the earth which are bounded severally by its five zones, the earth's diameter being 7912 miles, and the part of the diameter corresponding to each of the frigid zones being 327.19, to each temperate zone 2053.47, and to the torrid zone 3150.68.

Ans. Each frigid zone 1293793463.32, each temperate zone 55013912318.45, and the torrid zone 146717393786.26 cubic miles.

THE SPHEROID.

690. A SPHEROID is a solid which may be described by the revolution of an ellipse about one of its axes, which remains immovable.

An oblate spheroid is one described by the revolution of the ellipse about its minor or conjugate axis.

A prolate spheroid is one described by the revolution of the ellipse about its major or transverse axis.

PROBLEM XXIII.

691. To find the solidity of a SPHEROID.

Multiply the square of the axis of revolution by the fixed axis, and that product by 0.5236.

A full demonstration of this, which is based upon the principle that a spheroid is two thirds of its circumscribing

cylinder, would require a knowledge of Conic Sections, or of the Differential and Integral Calculi, with neither of which is the learner here supposed to be acquainted.

The relation, however, of the spheroid to its circumscribing cylinder, is that which the sphere sustains to its circumscribing cylinder (Prop. X. Bk. X.).

Now the area of the base of the cylinder is found by multiplying the square of the axis of revolution by 0.7854, and the solidity of the cylinder by multiplying that product by the fixed axis (Prop. II. Bk. X.). But the solidity of the spheroid is only two thirds of that of the cylinder; hence, to obtain the solidity of the former, instead of multiplying by 0.7854, we must use a factor only two thirds as large, which will be 0.5236.

2. Required the solidity of a prolate spheroid, whose fixed axis is 50 feet, and the axis of revolution 36 feet. 3. What is the solidity of a prolate, and also of an oblate spheroid, the axes of each being 25 and 15 inches?

Ans. Prolate, 2945.25 cu. in.; oblate, 4908.75 cu. in. 4. What is the solidity of a prolate, and also of an oblate spheroid, the axes of each being 3 feet 6 inches and 2 feet 10 inches?

5. Required the solidity of the earth, its figure being that of an oblate spheroid whose axes are 7925.3 and 7898.9 miles. Ans. 259774584886.834 cubic miles.

BOOK XIII.

MISCELLANEOUS GEOMETRICAL EXERCISES.

1. IF the opposite angles formed by four lines meeting at a point are equal, these lines form but two straight lines.

2. If the equal sides of an isosceles triangle are produced, the two exterior angles formed with the base will be equal.

3. The sum of any two sides of a triangle is greater than the third side.

4. If from any point within a triangle two straight lines are drawn to the extremities of either side, they will include a greater angle than that contained by the other two sides.

5. If two quadrilaterals have the four sides of the one equal to the four sides of the other, each to each, and the angle included by any two sides of the one equal to the angle contained by the corresponding sides of the other, the quadrilaterals are themselves equal.

6. The sum of the diagonals of a trapezium is less than the sum of any four lines which can be drawn to the four angles from any point within the figure, except from the intersection of the diagonals.

7. Lines joining the corresponding extremities of two equal and parallel straight lines, are themselves equal and parallel, and the figure formed is a parallelogram.

8. If, in the sides of a square, at equal distances from the four angles, points be taken, one in each side, the straight lines joining these points will form a square.

9. If one angle of a parallelogram is a right angle, all its angles are right angles.

10. Any straight line drawn through the middle point of a diagonal of a parallelogram to meet the sides, is bisected in that point, and likewise bisects the parallelogram.

11. If four magnitudes are proportionals, the first and second may be multiplied or divided by the same magnitude, and also the third and fourth by the same magnitude, and the resulting magnitudes will be proportional.

12. If four magnitudes are proportionals, the first and third may be multiplied or divided by the same magnitude, and also the second and fourth by the same magnitude, and the resulting magnitudes will be proportionals.

13. If there be two sets of proportional magnitudes, the quotients of the corresponding terms will be proportionals. 14. If any two points be taken in the circumference of a circle, the straight line joining them will lie wholly within the circle.

15. The diameter is the longest straight line that can be inscribed in a circle.

16. If two straight lines intercept equal arcs of a circle, and do not cut each other within the circle, the lines will be parallel.

17. If a straight line be drawn to touch a circle, and be parallel to a chord, the point of contact will be the middle point of the arc cut off by that chord.

18. If two circles cut each other, and from either point of intersection diameters be drawn, the extremities of these diameters and the other point of intersection will be in the same straight line.

19. If one of the equal sides of an isosceles triangle be the diameter of a circle, the circumference of the circle will bisect the base of the triangle.

20. If the opposite angles of a quadrilateral be together equal to two right angles, a circle may be circumscribed about the quadrilateral.

21. Parallelograms which have two sides and the included angle equal in each, are themselves equal.

22. Equivalent triangles upon the same base, and upon the same side of it, are between the same parallels.

23. If the middle points of the sides of a trapezoid, which are not parallel, be joined by a straight line, that line will be parallel to each of the two parallel sides, and be equal to half their sum.

24. If, in opposite sides of a parallelogram, at equal distances from opposite angles, points be taken, one in each side, the straight line joining these points will bisect the parallelogram.

25. The perimeter of an isosceles triangle is greater than the perimeter of a rectangular parallelogram, which is of the same altitude with, and equivalent to, the given triangle.

26. If the sides of the square described upon the hypothenuse of a right-angled triangle be produced to meet the sides (produced if necessary) of the squares described upon the other two sides of the triangle, they will form triangles equiangular with and equal to the given triangle. 27. A square circumscribed about a given circle is double a square inscribed in the same circle.

28. If the sum of the squares of the four sides of a quadrilateral be equivalent to the sum of the squares of the two diagonals, the figure is a parallelogram.

29. Straight lines drawn from the vertices of a triangle, so as to bisect the opposite sides, bisect also the triangle. 30. The straight lines which bisect the three angles of a triangle meet in the same point.

31. The area of a triangle is equal to its perimeter multiplied by half the radius of the inscribed circle.

32. If the points of bisection of the sides of a given triangle be joined, the triangle so formed will be one fourth of the given triangle.

33. To describe a square upon a given straight line.