652 THE POPULAR SCIENCE MONTHLY. 



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 erup- 

 tions 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 ex- 

 cessive 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 won- 

 derful and mysterious concealed in it as in anything else. Trans- 

 lated for The Popular Science Monthly from Die Gartenlaube. 



IN the interest of good roads, the watering carts of Maiden, Mass., are fur- 

 nished 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. 



