3oo POPULAR SCIENCE MONTHLY. 



tempestuous body. Further, the outrushing materials must acquire 

 the higher rotational speeds of the surface strata, and the inrushing 

 must lose their tangential momentum; and these can scarcely be in- 

 effective factors in the sun's circulatory system. 



The mechanical theory of the maintenance of at least a part of 

 the sun's radiation must be considered as a necessary consequence of 

 the law of gravitation — as unavoidably a consequence of that law as 

 precession is. Helmholtz computed that a contraction of the solar 

 diameter of less than 400 feet per year would suffice to maintain the 

 present rate of flow. Whether this is the sole source of supply is 

 uncertain, and very doubtful. The discovery of sub-atomic forces in 

 uranium, thorium and radium is of interest in this connection. These 

 radio-active substances have revealed the existence of intense forces 

 within the atom, long dreamed of by students of physics and chem- 

 istry, but never before realized. The energy radiated by an atom of 

 these substances is thousands of times greater than that represented 

 by the ordinary chemical transformations of equal masses of any 

 known element. Whether these forces are working within the sun, 

 prolonging its life many fold, and incidentally diminishing the re- 

 quired rate of Helmholtzian contraction, we do not know; but we are 

 not justified in treating gravitation as the sole regulator of radiation. 

 We are encouraged to this view by the fact that the age of the earth, 

 as interpreted by geology and biology, is many times greater than the 

 superior limit set by the gravitational theory. 



The dazzlingly brilliant photospheric veil which limits the depth 

 of our solar view is due, with no room for doubt, to the condensation 

 of those metallic vapors which, by radiation to cold space, have cooled 

 below their critical temperatures. These clouds form and float in 

 a great sea of uncondensed vapors, very much as do our terrestrial 

 clouds; but it seems probable that the process of formation is con- 

 tinuous and rapid; and that they are added to from above, or from 

 the interstices, and melt away from below. 



The sun spots are the most extensively studied and the least under- 

 stood of all solar phenomena. That they are large-scale interruptions 

 in the photosphere, and at the same time the most striking evidence of 

 atmospheric circulation, there can be no doubt. Observations made 

 near the sun's limb, to determine whether the spots are elevations or 

 depressions with reference to the photosphere, seem not to be reliable, 

 perhaps because of abnormal refractions in the strata overlying and 

 surrounding the spots. In the the earth's atmosphere, a high barom- 

 eter is the indication of descending currents, which generate heat by 

 compression and prevent cloud formation. Is not the umbra of a spot 

 an area of high pressure, which forces the solar atmosphere slowly 

 downward, preventing cloud formation in that area, but favoring the 



