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60.

assist in rendering the instruments visible on the return.

Some means will, of course, be necessary to check the velocity of the returning instruments. It might not appear, at first sight, that a parachute would be operative at a velocity of 10,000 ft/sec. or more; but it should be remembered that this velocity will occur in air of very small density, so that the pressure, or force per unit area of the parachute, would not be excessive; notwithstanding the high velocity of the apparatus. The magnitudes of the air resistance will, of course, be much larger than would be indicated from the values of [[underlined]] R [[/underlined]] in Tables V and VI, from the fact that, for motion with the parachute, the cross-section will be much larger in proportion to the mass of the rocket than for the cases presented in those Tables.

If the parachute is so large that the velocity will be decreased greatly when the denser air is reached, the descent will be so slow that finding of the apparatus will not be so easy as would be the case with a more rapid descent. For this reason, part of the parachute device must be lost automatically when the apparatus has fallen into air of a certain density; or else the parachute must be small enough to facilitate a rapid descent, with additional parachute devices rendered operative as the rocket reaches the ground. Such devices are not described in the present paper, but can be of simple and light construction.

The effectiveness of a parachute of even moderate size operating in a region where the density is small, may be demonstrated by the following concrete example. Suppose that an apparatus weighing one pound and having a parachute of one square foot area descends from the altitude, 1,288,000 ft. (over 200 miles), and does