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Show CANNON DALL. Measuring Its Velocity by Aid of Electrically Elec-trically Charged Screens. Measuring the velocity of n cannon ball mny seem to he u dlfllcult matter, but It Is really uno of thu simplest of tn tTKttsVsBLttHkJkli I 1 I'll 1 The Cannon and Targets. sclentlllc problems. The ncconipaii) Ing diagram i-hows the whole thing plainly. Somewhere In the psih of the pro Jectllu are placed two wlm scieens. marked C and D, sich formed of a ilmplo frame, ncros which a wire runs back and forth, forming part of an electric battery circuit In which an accurate tlincpltre, T, U also Inter posed. As the projectile, on being tiled from the gun pukm through the fccreen C, the circuit of which It Is a part U broken and the clockwork Is started, tho Index orlKlmtlly pobitlnx to zero. On packing through the other screen, D. the piojectlle lnterniHs thu icconil clicult, with the effect of Instantly In-stantly brlnKltiK the clock mechanism to a stop. Knowing, then, the exact distance between the two screens, usually several sev-eral hundred feet, it follows that hy ill i vldlng It l the number of seconds or j fractions of a wcoml murkud on the I clock dial. repn-KuntlnK the lime (hat has elapsed during the paw so of tho projectile from M-reen (' to screen I), wu obtain the number of fwit per no-mid no-mid that thu ripjutl was traveling. |