104 PROCEEDINGS OP THE CANADIAN INSTITUTE. 



oxygen and nitrogen which so largely predominate on the surface.* We now 

 know that the higher the clouds are the faster they move and del et Terre says 

 that the motion of cirrhus clouds is on the average 60 feet a second in latitudes 

 like our own and 45 feet within the tropics, while there are thus currents in the 

 upper air to the violence of which nothing indicates the limits. The word 

 violence I understand implies chiefly velocity and amplitude, for in highly 

 rarefied air, the force of such currents must not be likened to what we should 

 experience if there were at our level a constant gale of from 30 to 45 miles an 

 hour. From mountains and balloons those who frequent high altitudes have often 

 seen below them the upper surface of a layer of clouds, the existence of which 

 surface depends upon a delicate adjustment of heat and gravity. They have 

 described how huge billows will rise from the placid and shining cloud-layer and 

 sometimes subside as quickly as they arose. Ballons sondes and high-flying kites 

 have cai'ried instruments which show that there are frequent horizontal strata in 

 our atmosphere, and that the low barometer in one is seldom vertical to the low 

 in another, so that the lowest reading at a height of ten thousand feet may be 

 hundreds of miles distant from the lowest reading at the surface. And the 

 characteristics of these layers are very different. Thus ; the outer one, which we 

 never shall reach, must shade off" in temperature to the cold of space, dust and 

 moisture never reach it and its inferior surface is the upper limit to the lightest 

 possible cloud. Then comes the air of which the lower limit determines the snow- 

 line on our mountains. Lastly we may place the shell in which we live, within 

 which alone lightning flashes and rains fall, and there is enough moisture to inter- 

 pose a blessed screen against the terrible cold of a very few miles above. We will 

 not consider the terrestrial hydrosphere and lithosphere because there can be 

 nothing analogous to them in the solar orb, to which we will now turn. 



The first scientific conception I can find as to the physical nature of the sun is 

 that of Anaxagoras, who is reported to have said it was a red-hot stone, as large 

 as the Peloponnesus. A hundred years ago it was defined as a glowing solid 

 mass, stationary in the heavens. Even Sir John Herschell in the early edition of 

 his astronomy which I used when a school boy said " it is hardly possible to avoid 

 associating our conceptions of an object of definite globular shape and of such 

 enormous dimensions with some corresponding attribute of massiveness and 

 material solidity." A theory that it was liquid fire prevailed for a time. But it 

 seems to be regarded now as composed of incandescent gas, and I too believe that 

 the sun is a great globe of such vapours or gases, of which the visible outer 

 envelope is as tenuous as the smoke of a cigar. 



No sooner had Galileo turned his perspicillum on the sun than he perceived 

 its frequent spots, and it was his treatise Delle macchie solari which was the 

 ostensible cause of his disfavour with the papacy. Milton, who as a youth visited 

 him, has a half punning allusion to them : — 



. . . . " A spot like which, perhaps. 



Astronomer in the sun's lucent orb 



Through his glazed optic tube yet never saw." 



Their nature was mysterious then, and the question as to their cause and 

 nature is not yet surely answered. One plausible theory, which still holds a 

 certain sway, is Wilson's, who thought they were depressions in the luminous 

 solar envelope, through which the dark interior body of the sun became visible. 

 But out of hundreds of drawings, made with the utmost care and minutely 

 examined, less than one in three gives any countenance whatever to this view. 

 Were it true there should be a regular shading off' from the circumference of spots 

 to their centre, whereas there are only two well marked distinctions, viz., the 

 black looking umbra near the middle and the more lightly shaded penumbra 

 irregularly surrounding it. The way spots are usually drawn in astronomical 

 journals has become conventionalized ; radiations from the centre towards the 

 circumference or vice versa are rarely to be seen. Moreover, this hypothesis 

 assumes the interior layers to be less luminous than the exterior, which, as they 

 cannot well be cooler, is improbable. Another theory was that the spots are 

 scum or slag, floating on the surface of molten matter, but if the visible surface be 



* M. G. Heinrichs, Comptes Rendus de 1' Academic, August 20th, igoo. 



