Chambers's encyclopaedia; a dictionary of universal knowledge . temperatures ami pressures there prevailing,with the scale on whiili these must vary in shortintervals of time, will make vapours behave muchmore like terrestrial solids than like giu= as weknow them. The impact of a small jet of solarvap<mr would in fact be far more powerful thanthat of a projectile from a 100-ton g\in. Therapidity of these convection currents must there-fore be enormous, and a little careful watchingsoon shows that the whole solar surface is in astate of constant change. In certain regions of the photosphe
Chambers's encyclopaedia; a dictionary of universal knowledge . temperatures ami pressures there prevailing,with the scale on whiili these must vary in shortintervals of time, will make vapours behave muchmore like terrestrial solids than like giu= as weknow them. The impact of a small jet of solarvap<mr would in fact be far more powerful thanthat of a projectile from a 100-ton g\in. Therapidity of these convection currents must there-fore be enormous, and a little careful watchingsoon shows that the whole solar surface is in astate of constant change. In certain regions of the photosphere, between6 and . solar latitude, both north and south ofthe solar equator, large black spots are frequentlyobservefl. In size these vary from ,000 miles inlargest diameter to small black dots approachingin appearance the pores. The largest are easilyseen by the naked eye when fog or dark glass pro-tects it from the solar glare. The activityof their producing cause is subject to a considerablevariation. Schwabe of Dessau in announced. Fig- 2.—Sun-spots. From a photograph taken Febnmry i13, 1892, 9 hoars 47 minntes. By permission of theAstronomer-royal. The centre of the snns disc is Iat rt. ! the discovery of this important fact, giving about ten years its period. AVolf in ;2 correctedthis to 11 II This is generally accepted asthe mean period, but individual periods may varyfrom it The shortest periods are ithe most intense. There is an undoubted con- nection between this period and that of terres-trial magnetic phenomena. .Vurora and sun-spots and wane together, even in their smallerfluctuations. But the theory- that sun-spots depend for their frequency on the influence and position ofthe planets has had to be al>aniloned. spotsusually have three well-marked areas, distiniuishedby their diflerent degrees of blackness. The jim-nnibra forms the outer border of the spot, and isonly grayish compared with th
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