HARDWICKE'S SCIENCE-GOSSIP. 



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the telescope, we shall, on viewing the sun, be at 

 once struck by the peculiar mottled appearance of his 

 surface. This is always more or less visible, and is 

 due to intervening spaces of different degrees of 

 brightness. Nasmyth, in i860, considered that it 

 was caused by a multitude of interlacing masses 

 somwhat resembling willow-leaves in shape ; while 

 Huggins and others are in favour of the term granules, 

 or rice-grains, in describing these luminous particles. 

 Bright streaks or faculse are frequently seen in the 

 neighbourhood of the equatorial region. Much more 

 easy of observation are the dark spots first described 

 by Galileo. These vary very much in size, and not 

 unfrequently attain such enormous dimensions as to be 

 visible to the naked eye. The distance of the earth 

 from the sun is, it will be remembered, about 92,000,000 

 miles, and the real diameter of the latter rather more 

 than S66,ooo miles (though we have good grounds 

 for assuming that it is but a small star), and its ap- 

 parent diameter differs very little from half a degree. 

 It has been found that spots subtending an angle of 

 not less than 50 seconds of arc (which represents a 

 diameter of 24,000 miles) can be seen without 

 instrumental aid through a mist, or by the mere 

 intervention of a dark glass. Single spots have been 

 known to measure forty or fifty; thousand miles in 

 diameter, while many groups have been observed 

 covering areas of more than one hundred thousand 

 square miles. The region of spots is usually confined 

 to a broad space extending 35 on either side of the 

 sun's equator. From observations made by Schwabe 

 during the long period of forty-three years, not a 

 single clear day in which was allowed to pass without 

 a careful scrutiny of the solar surface by his " imper- 

 turbable telescope," and the counting of the number 

 of spots visible thereon, he came to the conclusion in 

 1844 that they were of a periodic character, and 

 attained a maximum once in about every ten years. 

 By a more rigid determination, Wolf, in 1852, found 

 that a period of ii'ii years more nearly satisfied the 

 conditions. What these spots are, what causes them, 

 and what effects their destruction — sometimes almost 

 momentary — we know not with any degree of 

 certainty. We do know that magnetic storms on the 

 earth are in some way connected with them. They 

 present the appearance of dark spots of irregular 

 shape, to which the name umbra has been given, 

 surrounded by a penumbra or lighter border. A 

 nucleus of still more intense darkness has sometimes 

 been detected within the umbra ; its blackness is 

 most probably not real, but merely the effect of 

 contrast. From the movement of spots, it has been 

 found that the sun rotates on its axis once in twenty- 

 five and a quarter days. 



When, during total eclipses of the sun, the dark 

 body of the moon comes between him and us, and 

 the intense light is thereby cut off, rose-coloured 

 prominences, or red flames, are seen at his edge. 

 Under the most favourable circumstances, the period 



of totality rarely exceeds a few minutes — too short a 

 time for a minute investigation of these phenomena. 

 It was, therefore, with no small amount of satisfaction 

 that astronomers learnt, in 1868, that Lockyer and 

 Jannsen had independently discovered a method by 

 which prominences could be observed on the un- 

 eclipsed sun. The spectroscope revealed the fact 

 that they are glowing masses of hydrogen, reaching 

 to enormous distances, occasionally extending from 

 the sun's surface for nearly a quarter of his diameter, 

 and presenting all sorts of fantastic shapes. Since 

 that time, improvements in instrumental means and 

 in the mode of observing have led to the knowledge 

 that, by widening the slit of the spectroscope, the 

 flames themselves may be seen at any time at the 

 limb of the solar disc. There is no doubt that a very 

 close connection exists between spots, facula?, and 

 prominences, and one in which it will be found that 

 electricity plays a very prominent part. 



If the sun were a thin shell (and, as a matter of 

 fact, we know that not only that body, but all the 

 planets exterior to the orbit of Mars, are not com- 

 posed of solid materials, as our earth is,) and it were 

 possible to place the earth at his centre, there would 

 be room for the moon, with its diameter of 2165 

 miles, to revolve round it at its present distance, and 

 leave a marginal radius of not far short of 187,000 

 miles. As our nearest neighbour, and, in a very 

 great measure, the cause of the tides, our moon and 

 its motions are of great interest. As a telescopic 

 object it is a dead world, subject to no change, and, 

 consequently, we are enabled to study its surface at 

 leisure, more especially as, the period of its rotation 

 on its axis and of its revolution round its primary 

 being performed in similar time, the same face is 

 always presented to us. The observer will immedi- 

 ately notice that the moon's comparative proximity 

 to us necessitates an alteration in the position of the 

 eye-piece, in order to obtain distinct vision. To the 

 naked eye the lunar surface presents a somewhat 

 mottled appearance, due to the unequally reflective 

 quality of its material. The moon, in its varying 

 path in the heavens, is constantly passing between us 

 and some star or other. During these occultations 

 the light of the star is instantaneously snuffed out, as 

 it were, when overtaken by the moon's limb, and its 

 re-appearance is as sudden. This could not be the 

 case if there were a lunar atmosphere of even extreme 

 tenuity ; consequently no water can exist there, 

 although the grey plains formerly received, under a 

 false impression, and still retain, the name of seas. 

 When about to observe the moon, especially with a 

 large telescope, it will be advisable to use a light 

 neutral-tint sunshade (Venus, too, is best seen thus in 

 the morning or evening), or a power sufficiently high 

 to reduce the glare. On its surface are plains and 

 valleys, mountain-chains and craters, isolated hills 

 and ridges, rills, faults and clefts. The shadows cast 

 by the mountains and other formations give us a 



