COSMOGONY—JEANS $55 
thinking in terms of motions rather than of distances. Light, 
which can circle the earth seven times in a second, would move in 
our model with a speed rather below that at which a blade of grass 
grows in the spring. On this scale the whole universe will be 
represented by a sphere of the size of our earth, the star cloud of 
which our sun is a member will be an island of about the size of 
Yorkshire, while the big Andromeda nebula will be rather larger 
than the Isle of Wight, although with very ill-defined boundaries. 
The whole solar system in this model can be easily covered by a 
grain of sand, while our earth, now shrunk to less than a ten- 
millionth of an inch in diameter, is hardly larger than a single 
molecule in this grain of sand. 
Such is the universe which the astronomer hands over to the 
cosmogonist for interpretation. The cosmogonist, accepting the 
universe as it is, must try to discover why it is thus and not other- 
wise. What the astronomer regards as a compilation of observed 
facts is for the cosmogonist the last link in a long chain of processes, 
a crosscut through the warp and the woof of cause and effect. 
While the astronomer is satisfied if he can see the universe as it is, 
the cosmogonist must ever strive to see it as it has been and as it 
will be. Just as one of the astronomer’s main problems is to assign 
limits to the universe in space, so one of the main problems for the 
cosmogonist is to assign similar limits in time. 
There must be such limits. The universe can not go on for- 
ever as it now is, and neither can it have existed in its present 
condition from all eternity. KEvery star is continually radiating 
energy away into space, and we have no knowledge of any appre- 
ciable part of this radiation coming back or of the stars replenishing 
their sources of energy in any way. The universe is running down 
like a clock which no one winds up. 
The sun has some ten thousand million million million square 
inches of surface, and every square inch is radiating away energy 
at a rate which represents the energy output of a 50-horsepower 
engine. If this energy were supplied to the sun from a power 
station, coal would have to be burned at the rate of about a million 
million million tons a minute. This makes it clear that the sun’s 
energy can not, as was at one time thought, originate in the combus- 
tion of the sun’s mass. At a later date Meyer suggested that the 
sun’s energy might be continually replenished by the infall of meteor- 
ites, while Helmholtz subsequently propounded his well-known 
contraction theory, according to which the energy of the sun’s radia- 
tion is provided by the falling in of the sun’s mass under his own 
gravitational attraction. Both these theories implied limits to the 
duration of the sun’s radiation, and both limits were far too short 
to accord with known facts. Meteorites could not have been falling 
