Page:Popular Science Monthly Volume 45.djvu/672

652 participate, like earthly things, in the revolution around the axis, but remain fixed among the stars through the night. When near enough to the earth they can be seen with the naked eye as luminous clouds, long after sunset, till they are covered by the earth's shadow.

The presence of dust in planetary space is not strange. In the midst of it is the sun, the surface of which is like an immense volcano. We can only ask how the dust clouds of the solar eruptions can be diffused in space, against the attraction of the sun. An answer to this question is afforded by the electro-magnetic theory of light, and we can rely upon it because the theory has been confirmed by experimental demonstration. It teaches that the lighter undulations of the ether are of an electrical nature, and that consequently light exerts a pressure on all bodies upon which it falls. The illuminated body is repelled from the source of the light. We have also learned the amount of this pressure. It is so small that the scale of the most sensitive balance is not moved by it when the clear sunshine falls upon it from above; but it increases with the extent of the surface exposed to the light. Let us now suppose a body isolated anywhere in planetary space. It is subject to general attraction and is drawn toward the sun. The force with which the light of the sun repels it is slight as compared with the attraction. Let us imagine this body divided into smaller and smaller fragments. It then offers the sun a larger and larger surface, and in the same measure the force increases with which all the parts collectively are repelled from the sun. The amount of attractive force is, on the other hand, not changed, for it depends upon the mass of the body, and that has not been altered. It will be seen that the division of the body has only to be carried far enough for the repulsive force ultimately to exceed the attraction. Calculation shows that this is already the case when the body is changed into a dust cloud of not excessive fineness. Such a dust cloud will be no longer attracted toward the sun, but will be driven away by its light. It will be like the comets' tails, which consist chiefly of dust, radiate from the nuclei, and are always turned away from the sun.

Thus, even insignificant, common dust has its considerable part in the processes of Nature; and there is as much of the wonderful and mysterious concealed in it as in anything else.—Translated for The Popular Science Monthly from Die Gartenlaube.

the interest of good roads, the watering carts of Malden, Mass., are furnished with broad tires, of which the forward pair are set nearer together than those of the rear, so that the track of the former is just inside of that of the latter. The carts thus serve as rollers as well as for their primary purpose.