[5] That results from the very strong refraction which light undergoes in the atmosphere of Venus when this planet is seen in front of the sun’s edge during the so-called Venus transits.
[6] The moon strongly, and more than any other agent, influences the tides. Apart from this effect the position of the moon has only a feeble influence upon the air pressure and upon atmospheric electricity and terrestrial magnetism. The influence of the stars is imperceptible.
[7] One centimetre of water contains 470 billions of these spheres. Such a little drop of water, again, contains 96 millions of molecules, and there are probably organisms which are smaller than these drops. Compare the experiments with ultra-microscopic organisms by E. Raehlmann, N. Gaidukow, and others.
[8] The designations "hard" and "soft" streams of solar dust correspond to the terms used with regard to kathode rays. The soft rays have a smaller velocity, and are therefore more strongly deflected by external forces, as, for instance, magnetic forces.
[9] The reason is that in the southern district only very few, and chiefly the most intense, auroras are recorded. If we observe very assiduously in a large country, and conduct the observations at different spots, we shall find polar light almost every night. This consideration partly wipes out the just-mentioned differences.
[10] The very highest strata of our atmosphere (at levels of from 20 to 80 km., 15 to 50 miles) may perhaps form an exception. The luminous clouds which were observed in the years 1883-1892 at Berlin (after the eruption of Krakatoa), and which were floating at a very high level, showed a drift with regard to the surface of the earth opposite to the drift of the cirrus clouds, which are directed eastward.
[11] According to Memery (Bull. Soc. Astr., March 7, 1906, p. 168) an instantaneous rise of temperature is observed immediately when a sun-spot is first seen, and the temperature sinks again when the sun-spot disappears.
[12] Stars are classified in magnitude, the order being such that the most luminous stars have the lowest numbers. A star of the first magnitude is 2.52 times brighter than a star of the second magnitude; this, again, 2.52 times brighter than a star of the third magnitude, and so on. All this from the point of view of an observer on the earth.
[13] When, standing on a station platform, we watch an express train rushing through the station, the pitch of the engine whistle seems to become higher as long as the train is approaching us, and deeper again when the train is moving away from us. The pitch of a note depends upon the number of oscillations which our ear receives per second. Now, when the train is fast approaching us, more vibrations are sent into our ear than when the train is at a stand-still, and the pitch, therefore, appears to become higher. The same reasoning holds for light waves, of which Doppler, of Prague (Bohemia), was in fact thinking when first announcing his principle in 1842. The wave-length of a particular color of the spectrum is fixed with the aid of some Fraunhofer line characteristic of a certain metal. If we compare the spectrum of a star and the spectrum of a glowing metal, photographed on the same plate, the stellar lines will appear shifted towards the violet end (violet light is produced by nearly twice as many vibrations of the ether per second as red light) when the star is moving towards us in the line of sight. This principle has successfully been applied by Huggins, H. C. Vogel, and others, for determining the motion of a star in our line of sight. When a star is revolving about its own axis, the equatorial belt will seem to come nearer to us (or to recede from us), while the polar regions will seem to be at a stand-still; the lines will then appear oblique (not vertical). In this way Keeler proved that the rings of Saturn consists of swarms of meteorites moving at different velocities in the different rings.—H. B.
[14] One light year corresponds to 9.5 billion kilometres, and it is the distance which the light traverses in the course of a year.