1887.] on the Sun's Heat. 18 



scientific investigators who are devoting themselves to advancing 

 the science of solar physics, consider the easily understood ques- 

 tion, What is the temperature of the centre of the sun at any time, 

 and does it rise or fall as time advances ? If we go back a few 

 million years to a time when we may believe the sun to have been 

 wholly gaseous to the centre, then certainly the central tempera- 

 ture must have been augmenting ; again, if, as is possible though not 

 probable at the present time, but may probably be the case at some 

 future time, there be a solid nucleus, then certainly the central 

 temperature would be augmenting, because the conduction of heat 

 outwards through the solid would be too slow to compensate the 

 augmentation of pressure due to augmentation of gravity in the 

 shrinking fluid around the solid. But at a certain time in the 

 history of a wholly fluid globe, primitively rare enough through- 

 out to be gaseous, shrinking under the influence of its own gravi- 

 tation and its radiation of heat outwards into cold surrounding 

 space, when the central parts have become so much condensed as to 

 resist further condensation greatly more than according to the 

 gaseous law of simple proportions, it seems to me certain that the 

 early process of becoming warmer, which has been demonstrated by 

 Lane, and Newcomb, and Ball, must cease, and that the central 

 temperature must begin to diminish on account of the cooling by 

 radiation from the surface, and the niixing of the cooled fluid through- 

 out the interior. 



Now we come to the most interesting part of our subject — the 

 early history of the Sun. Five or ten million years ago he may have 

 been about double his present diameter and an eighth of his present 

 mean density, or • 175 of the density of water ; but we cannot, with 

 any probability of argument or speculation, go on continuously much 

 beyond that. We cannot, however, help asking the question, What 

 was the condition cf the sun's matter before it came together and 

 became hot? It may have been two cool solid masses, which collided 

 with the velocity due to their mutual gravitation ; or, but with 

 enormously less of probability, it may have been two masses colliding 

 with velocities considerably greater than the velocities due to mutual 

 gravitation. This last supposition implies that, calling the two bodies 

 A and B for brevity, the motion of the centre of inertia of B rela- 

 tively to A, must, when the distances between them was great, have 

 been directed with great exactness to pass through the centre of 

 inertia of A ; such great exactness that the rotational momentum, or 

 " moment of momentum," * after collision was no more than to let 



* This is a technical expression in dynamics which means the importance of 

 motion relatively to revolution or rotation round an axis. Momentum is an 

 expression given about a hundred and fifty years ago (when mathematicians and 

 other learned men spoke and wrote Latin) to signify translational importance of 

 motion. Moment of a couple, moment of a magnet, moment of inertia, moment 

 of force round an axis, moment of momentum round an axis, and corresponding 

 verbal combinations in French and German, are expressions which have been 



