Skpt. 4, 1885.] 



KNOW^LEDGE 



charge collects at A, and additional negative is driven 

 into the leaves, whence the previously existing divergence 

 is increased. An increased divergence then proves that 

 the unknown charge is similar to the known charge 

 in the leaves. The falling together (or collapse) of the 

 leaves may indicate an opposite (in this case positive) 

 charge on the body being tested, but there is a little 

 chance of our being deceived in this case, as the approach 

 of a neutral body might produce a like effect, for the 

 charge in the leaves would speed to the assistance of the 

 similar charge on A in its efforts to electrify the pre- 

 viously unelectrified or neutral rod. This may be proved 

 by bringing the hand near A. 



With a ball or plate at A (instead of a metallic point, 

 or the end of the wire) the charge on the leaves may be 

 retained for a considerable time, but this advantage is 

 lost if there are any points on the metallic system, or if 

 the air confined in the jar has not been thoroughly dried. 

 The necessity for dry warm air is apparent. There is 

 some danger that, in the event of the charge on the rod 

 under examination being exceptionally strong, the leaves 

 will be made to take ujj positions so far divergent that 

 they enter into contact with the surface of the glass. 

 Now, the presence of a charge at L L indicates the ability 

 to induce a charge on the surface of F. This certainly 

 takes place when mutual attraction between this charge 

 and that on L L sets in. If the surface of F is but a 

 poor conductor, the leaves are liable to stick. This is 

 overcome by gumming or pasting two strips of tinfoil on 

 the inside of the glass, as K K (Fig. 59), and placing 

 them in connection with the earth. The charge upon 

 L L readily produces an opposite charge upon K K, the 

 similar charge pushing its way into the earth. The diver- 

 gence of L L is accordingly increased, and possibly L L 

 comes into contact with K K, in which case neutralisa- 

 tion immediately ensues, and the leaves fall together 

 uninjured. The metallic shield, G (Fig. 59), is to 

 prevent the electrified rod exerting any direct inductive 

 effect upon L L. The function of the sulphuric acid, 

 with which the pumice-stone in H is saturated, is to 

 absorb any atmospheric vapour that may be present. 



The objections to the form depicted in Fig. 60 are 

 apparent. In the first place, no tin-foil strips can be 

 utilised, and, in the second place, it is impossible to 

 remove any atmospheric moisture, dirt, &c., which may 

 have been left behind in sealing up. We will next turn 

 our attention to an instrument the cost of the materials 

 for which shall not exceed threepence, but which, never- 

 theless, will answer our every purpose. 



THE GREAT RED SPOT ON JUPITER. 



By Richard A. Proctor. 

 ( Continued from p. 157.) 



LET us look into this matter a little more closely : 

 and fir.st, let us ask if anything akin to the diffi- 

 culty thus recognised in the case of Jupiter (and also in 

 that of Saturn) exists elsewhere. 



Now in the case of the sun wc have an orb which is 

 probably in large part gaseous. We certainly have, 

 t'isibly, a gaseous rcgicui tlnmsiinds of miles in depth, 

 even estimating the depth <<n\y from (lie \ isible surface 

 of luminous cloud whieli we eiill the plictespliere. And 

 in tlu! .sun's case the attraction of gravity on the atmo- 

 Bpluric region thus recognised, is ten or twelve times 

 greater than the attraction on the atmosphere of Jupiter. 



Therefore we have in the sun's case a much greater 

 difficulty than in the case of Jupiter or Saturn. 



It is true that the intense heat pervading the whole 

 frame of the sun suggests a way of meeting the difficulty 

 which does not at first sight seem available in dealing 

 with the giant planets. The laws which connect density 

 and pressure at ordinary temperatures and at ordinary 

 pressures may probably fail altogether where the tem- 

 peratures are so high and the pressures so enormous as 

 they must be throughout the whole frame of the sun. 

 We may say, indeed, as I have elsewhere shown, re- 

 specting the outer parts of the sun we see, what 

 Professor Young said of the usually unseen corona, that 

 if the term atmosphere be understood as we understand it 

 when speaking of our own air, the gaseous regions forming 

 the parts of the sun next within the photosphere do not 

 form an atmosphere at all. Here are his remarks in regard 

 to the corona, each one of them being fully applicable 

 to the gaseous envelopes within the visible surface of the . 

 sun : — " Granting for the moment that the corona is in 

 part and largely composed of an envelope of exceedingly 

 rare gaseous matter around the sun, — then we may call it 

 an atmosphere, because being gaseous and attached to a 

 cosmical body, it bears to that body a relation analogous to 

 that borne by our atmosphere to the earth itself. So far 

 the term is a proper one. But now further, and on the con- 

 trary, the term ' atmosphere ' carries with it to most 

 persons certain ideas as to the distribution of temperature, 

 density, (fee, in its different parts, which are based on the 

 fact that our terrestrial atmosphere is nearly quiescent 

 and in static equilibrium \mder the force of gravity, 

 with a temperature not more than two or three 

 hundred degrees above the absolute zero, while the 

 density of the portion accessible to human observation 

 is very considei-able. On the sun the conditions are 

 immensely, and almost inconceivably different, so that 

 the term ' atmosphere ' becomes a very misleading one. 

 There the equilibrium, so far as there is any, is dyna- 

 mical, not statical, and the density, temperature, and 

 condition of the gaseous substance is far more nearly 

 that of the residual gas in a Crookes's vacuum tube 

 through which an induction coil is sending electrical dis- 

 charges ; so different from that of ordinary air that 

 Crookes thought he had found a fourth state of matter, 

 bearing some siieli i-elition to the gaseous state as the 

 gaseous does tn tlie lli|ui'l." 



That this is so In re-;ir(l to the sun is shown at once 

 if we remember tliM ilir j reat openings we call spots 



10,000 miles l.elnu llo '; ' "aifm-e. Xo« tlic 



strength and l.rea.lil. . : i- ■ ■ :' -■ n liiu s se, ,, n, the 

 spectrum of tlie mi.:- - : ::v~ 1 i' .mes sliou il,:,t ilie 



the 



l],r 



ity at the sun's visible surface is 27 

 earth's, we find that at a depth of 



.el.iw the sun's apparent surface at- 



be twenty doublings of pressure, r.iisiug the a.!,-) fioui 

 the millionth part of our air's to someuhai lu. )- ihan 

 equality with the density of our air ('J duuh!. .1. i hut 

 double doubled, and so on, to 20 doubiiugs, giving 

 1,048,570). In the next 2^ miles the pressure wouldbe 

 increased more than a millionfold, — always assuming 



