produced perhaps by the disruption of a sun that has grown cold, the individual blocks would tend to aggregate by gravitational attraction, and directly one little aggregate became formed it would act as a centre of attraction for the other outlying fragments, and thus it would grow rapidly.
The central point of the original swarm of meteorites is represented now by the sun, the smaller aggregates by the planets, and the original fragments by the comets and shooting stars which still are being attracted by the larger aggregates. The theory has been tested mathematically by F. R. Moulton, and is dynamically possible. If we accept it we must be prepared to explain all our evidence without the help of a hot interior of the globe.
The Temperature Gradient in the Earth's Crust. — On the Nebular Hypothesis the interior of the globe is very hot, and therefore the crust resting upon it should show evidence of this by becoming warmer the deeper we bore into it; and this is actually the case. A great many measurements in differents parts of the globe have been made with a view to ascertaining the average rate of increase as one goes downwards, and one degree of Fahrenheit for every sixty feet seems to be in round numbers a fair average. That is to say, below the surface layer, the temperature of which varies according to the heat it receives during the summer and winter from the sun's rays, there is a continuous rise of temperature as far as we have been able to penetrate the earth's crust. If the surface layer is at a temperature of 60 F., 60 ft. below the bottom of this layer the temperature will be 61 F., 600 ft. below, 70 F., and 6000 ft. below, 160 F., and so on. The variations in temperature gradient at different places, however, is very great, ranging from an in-
