UNIVERSE
185
UNIVERSE
edge of comets the argument can be made stringent.
More than three hundred comets have their orbits well
determined. Over two hundred of them have passed
the ecliptic within the earth's orbit, and some, hke
Halley's comet at its last appearance, almost in line
between sun and earth. Most of the comets, includ-
ing Halley's, come to us from distances beyond the
orbit of Neptune. Now, coniiiutation shows that
they all have their common focu.s in the sun and that
the earth is, as a rule, outside their orbits. In the case
of Halley's comet the earth was, at one time, even on
the convex side of the orbit. The mechanical conclu-
sion is as follows: If, without any regard to the
earth, the comets obey the sun, the earth must do the
same.
(2) The daily rotation of the earth around its axis is demonstrated in many wavs. Once the annual rev- olution is proved, the daily rotation becomes a matter of course. If the earth has not the power to swing the sun around its own centre once a year, it will be far less able to do so in one day; and if it cannot swing around one sun, what could it do with the countless suns of the universe? Yet, we have direct and special proofs of the diurnal rotation. They aU rest on mechanics, partly celestial, partly terrestrial. Celestial mechan- ics has turned into proofs what formerly seemed to be difficulties. This occurred in the case of stellar parallaxes, the absence of which had been objected by Ptolemy, and the existence of which was shown by Bes-sel. The precession of the equinoxes also has changed its role. Laplace showed it to be due to the action of the sun on the protuberant equatorial re- gions of the rotating earth. The similar result of the action of the moon upon the earth is called nutation. Laplace's demonstration was ba.sed upon the flatness of the earth, which had been measured in the seven- teenth century, and was also thcorelically deduced by him from the existence of centrifugal force. We have here a complex reverse of roles. The conse- quences of centrifugal force, so strongly urged against diurnal rotation by Ptolemy, turned out to be the cause of precession, known to Hipparchus, and of several phenomena, discovered only after the time of Copernicus. Precession was still a matter of special difficulty to Copernicus, and the one of the three terrestrial motions that he could not explain. To him it was the resultant of two annual, slightly differ- ent, conical rotations of opposite direction, to which no cause could be assigned.
So much about the proofs from celestial mechanics. There are others, by means of instruments, so-called laboratory experiments. They commenced imme- diately after the time of Clalilei and seem to have received the impul.se from his trial. The experiments may be classified chronologically in five periods or groups. From 1640 to 1770 they were crude trials without result. The years from 1790 to 1831 were a period of experiments with falUng bodies. The twenty years from 18.32 to 18.52 were a time of pendu- lum experiments. Then followed a period, 18.52-80, of experiments with more elaborate apparatus; and the last, since 1902, may be called that of modern methods.
(a) The first period is represented by the names of Calignon, Mersenne, Viviani, and Newton. Calig- non (1643) experimented with plumb lines, without knowing what their variations should tell. Mersenne (1(543) had pieces of artillery directed to the zenith, rightly ex-pecting a westerly deviation of the balls. Fourault's pendulum experiment was materially fore- stalled by Viviani at Florence (1661) and I'oleni at Padua (1742), but was not formally understood. The ea.sterly deviation of falling bodies was expHcitly announced by Newton, but unsuccessfully tried by Hooke (1680). Galilei had alluded to it before, in his "Dialogo" fOpere, VII, 1897), in a contradictory manner. In one place (p. 170) he denied the possi-
bility of the experiment, in another (p. 259) he
affirmed it. Lalande missed the opportunity of first
making Newton's experiment at the Paris observatory.
The honour was reserved to Abbate Gughelmini.
(b) The second period comprises the experiments with faUing bodies, made by Giiglieliuini at Bologna (1790-2), by Benzenberg at Hamburg (1S02) and Schlebusch (1804), and by Reich at Freiburg (1831). The general drift of )li(> balls towards the east side of the meridian was unniistakiiblc. It proved the rota- tion of the earth from west to cast, but only in a quali- tative manner. Quantitative proofs were obtained in the next period.
(c) Three kinds of pendulum experiments filled the third period. The horizontal pendulum was invented and tried by Hengler, in 1832, fur the effects of the centrifugal force. The instrument is still waiting for a more delicate manipulator. Foucault's vertical pendulum dates from 18.51, and was tried first in a cellar, then in the Paris Observatory, and last in the Panth6on. The deviation of the pendulum from the original vertical plane was clockwise, as expected by Foucault, but no quantitative measures were ever published by him. They were made in many places, chiefly in large cathedrals. The best results known are those of Secchi in Rome (1851) and of Garthe in Cologne (1852). Secchi ex-perimented in San Ignazio, in presence of many Italian scientists, and Garthe in the cathedral, before Cardinal Geissel, royal princes, and numerous spectators. The coun- terproof in the southern hemisphere, where the devia- tion of the pendulum must be counter-clockwise, has not been made to this day. The attempt at Rio de Janeiro (1851) cannot be regarded as .such. A conical pendulum was set in motion by Bravais in the same meridian room of the ob.servatory and in the same year as the vertical pendulum of Foucault. The ex- periment had the advantage of being reversible. S%vinging clockwise, the pendulum apiicared to move faster than in the opposite sense, for the reason that the theodolite, in which it was observed, followed the rotation of the earth. Two pendulums used simul- taneously, and moving in opposite directions, yielded the correct value of the diurnal rotation within a tenth of one per cent, a result never reached by Fou- cault's pendulum.
(d) The second half of the nineteenth century, the fourth period, is remarkable for complicated experi- ments and profound theories. The instruments were the gyroscojie and the compound pendulum. The invention of the former is due to Foucault, and fur- nished a new proof of the diurnal rotation. It w.as constructed by him in three forms: the universal, the vertical, and the horizontal gyroscope, so called according to their degrees of freedom. The vertical gyroscope was perfected by Gilbert (1878) into his barogyroscope, while the horizontal gyroscope was lately introduced on warships as an astronomical compass. The proofs of F'oucault and Gilbert could only be qualitative, for want of electric motors. The delicate experiment made in 1879 with the compound pendulum by Kamerlingh Onnes, comprises those of F'oucault and Bravais as special cases, and in general all the movements between the plane and the circular pendulum vibrations (sec "Specola Vaticana", 1, 1911, Appendix 1).
(e) The fifth and last period of experiments fails within the twentieth century and presents no less than four proofs, all widely different among themscb'cs. The difficult experiment with falling bodies was brought within the walls of the physical laboratoiy by E. H. Hall in 1902. Under improved facilities, a fall of only twenty-three metres showed the easterly deviation better than all the preceding trials with heights from three to seven times as large. In 1904 the gyroscope was made to yield quantitative results by Foppl. An electric motor gave to a double wheel
