Page:Popular Science Monthly Volume 51.djvu/697

Rh preceded by condensed gaseous waves. We can not, therefore, doubt that the cushion of greatly compressed air in front of the projectile causes considerable delay in its progress, and consequently a great beating of the ball. We know that this takes place with aëroliths which become incandescent and burst in flying through our atmosphere.

Melsen's experiments led me to suppose (in 1874) that the obstruction and heating of a projectile passing through the air might be notably diminished by driving a narrow and slightly conical channel through the ball and slipping into it a metallic obturator to fit it. "In this way," I said, in my lectures on thermodynamics, "the ball might be discharged without letting more gas escape than usual; once out of the chamber, it would condense the air in front of it, while the air behind it would be extremely rarefied. A difference of pressure would immediately be produced sufficient to force the conical tampon out of the projectile, and after that there would be no more projectile-air pressure." Under these conditions, I said, the velocity of projectiles could be kept up for much greater distances, and the heating would be considerably less. I was not so situated that I could verify these views by experiment; but the principle was applied about two years ago in Germany, in the Hebler Kruka ball, the axis of which is pierced with a small cylindrical channel, enlarged behind so as to be funnel-shaped, and closed with a small plug—the very device I had imagined twenty years before—which, when fired from a cannon, behaved just as I supposed my perforated ball would do.—Translated for the Popular Science Monthly from Ciel et Terre.



POTS or groups of spots were seen more or less distinctly upon the sun previous to the invention of the telescope. The observations are described under various forms, first among which may be mentioned obfuscations or obscurations of the sun. At other times they were believed to be passages of Mercury in front of the sun, as in 807, a date mentioned by the historians of Charlemagne, and on the 28th of May, 1607, when even Kepler was deceived. In 859 Alkindi thought he observed a transit of Venus; but the black object he saw on the sun's disk was only a spot large enough to be perceived by the naked eye.

Observations of obscurities or spots on the sun have been made in China at different epochs, the most ancient one dating from the year 301. Between that date and the beginning of the