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33.

of any successive up-and-down rushes of gas.

[[underlined]] SPRING IMPULSE METER. [[/underlined]]

A section of the spring impulse-meter is shown in Fig. 20(c). The apparatus consisted of an aluminum disk [[underlined]] D [[/underlined]], cemented to a lead rod, [[underlined]]L[[/underlined]], of combined mass 5.2956 gms. supported by a fine brass spiral spring, [[underlined]] S [[/underlined]]. The disk, [[underlined]] D [[/underlined]], was of a size sufficient to slide easily in a glass tube, [[underlined]] G [[/underlined]]. The upper end of the spring protruded through a small hole in the glass tube, and was fastened at this point by de Khotinsky cement; it thus being easy to make the top of the lead rod level with the zero of a paper scale, [[underlined]] K [[/underlined]], pasted to the outside of the glass tube. A piece of white paper placed behind the tube, [[underlined]] G [[/underlined]], made the motion of the lead rod [[underlined]] L [[/underlined]] very clearly discernible.

This impulse-meter was placed in a hole in the upper cap of the 12-inch pipe of the cylindrical tank, at [[underlined]] D [[/underlined]], Figs. 17 and 18, the same distance from the wall of the 12-inch pipe as the center of the 3-inch pipe. It projected one inch through the 12-inch cap which was practically the same as the distance the 3-inch pipe projected. The tube, [[underlined]] G [[/underlined]], was kept in position in the cap by being wrapped tightly with insulating tape, the joint being finally painted with the wax already described.

The theory of the spring impulse-meter is given in Appendix D, p. 76, where [[underlined]] Q [[/underlined]] is the ratio already defined in connection with the direct-lift impulse-meter. There are two reasons why the ratio, [[underlined]] Q [[/underlined]], obtained in the Appendix should be larger than the true percentage at the top of the 3-inch pipe. In the first place, friction in the 3-inch pipe will decrease the velocity of the rebounding gas; and further, the disk, [[underlined]] D [[/underlined]], Fig. 13 is fairly tight-fitting in the glass tube, [[underlined]]G [[/underlined]], whereas