374 
[June, 
The Origin of Falling Motion. 
the much greater effect of the solar radiations. But in the 
earth’s interior it may possibly produce important results. 
The variation in the earth’s internal temperature, through 
loss by conduction, is exceedingly minute. But we have 
here a source of a considerable increase during six months 
of the year, and a like decrease during the succeeding six 
months. 
These daily variations cannot be lost by radiation, but 
must accumulate, so that the temperature of internal water 
must vary 30° yearly, of mercury 900°, and of other sub- 
stances in like manner. Although we do not know what 
results are likely to arise from such an annual variation in 
temperature, yet it is very possible that these results may 
be of an important character. 
In the case of a planet of short period and great simplicity 
of orbit, such as we have in the planet Mercury, the effeCts 
resulting from this cause must be much greater than in the 
earth. It, indeed, must produce a marked effeCt on the 
surface temperature of Mercury, and an annual variation 
sufficient to partly neutralise the variations in the amount 
of solar heat upon this planet. 
An interesting conclusion from the hypothesis here ad- 
vanced is in regard to the simple and natural method in 
which one mode of motion becomes converted into another. 
The change from heat vibration into mass motion needs no 
special machinery and no difficult transfer of energy. 
Motion seems to be constantly at the command of attraction. 
The least definite pull in any fixed direction, if unresisted 
by opposing energy, at once converts heat motion into mass 
motion. This latter, in its turn, is persistent until resisted, 
when it immediately becomes converted into the independent 
movement of particles. 
The change from electricity to heat is probably as simple 
in its nature. The impelling cause, in all cases, seems to 
be some variation in attractive conditions, to which the 
moving particles instantly respond, their modes of motion 
becoming special results of the modes of attraction. 
And as there is but one motion, so there is, in all proba- 
bility, but one attraction. Gravitation, chemism, and mag- 
netism are probably modes of attraction, as heat, electricity, 
and mass movement are modes of motion. The different 
forms which these assume very likely result from the dif- 
ferent relations of position assumed by the particles of 
matter. It is probable, also, that molecules have special 
relations of position between their constituent parts, and 
that their outward attractions become specialised in 
