a certain ratio; and, if 16 be called 1, 48 will be 3, 80 will be 5, 112 will be 7, and 144 will be 9, etc. These distances may then be expressed as falling distances, thus, 1, 3, 5, 7, 9, etc. And, keeping in mind that the horizontal velocity remains uniform, that is 48 ft., i. e., 3 × 16 ft. in a second, we have two kinds of dimensions at right angles to each other, from which to make the curve. This curve is called a parabola.

442. Can you construct a parabola?

When these distances, instead of being written down as the separate result of each second's action, are successively added to show the combined results, we have for

From this it will be seen that the distance

fallen is as the square of the time, i. e., in 6

seconds the distance fallen will be 62 x 16 ft. = 36 × 16 ft. = 576 ft.

443. Required the distance a stone falls in half a second.

444. Required the distance a stone falls in 21 seconds.

445. Can you show that there are two kinds of quadrilaterals in which the diagonals must be equal, two kinds where they may be equal, and two kinds where they cannot be equal?

446. Can you make in card a tetrahedron whose four surfaces shall be unlike in form?

9

THE END.