408 ANNUAL OF SCIENTIFIC DISCOVERY. 



true, the orbits during several hundred years would all pass very near the 

 point where the explosion occurred, but after thirty thousand years the 

 orbits would have changed so much that no trace of this intersection would 

 be left. Consequently, if this explosion occurred at any time within the last 

 three millions of years, the fact that the orbits do not now intersect is no 

 argument against Gibers' hypothesis. 



The best way to treat the problem was to make actual calculations as exact 

 as possible of the positions of the orbits during all time. We can then easily 

 see whether they could have ever intersected. These calculations were 

 made by Mr. N. for the four asteroids, Vesta, Metis, Hygea and Parthenope. 



If the changes we now see going on in the orbits of the planets were 

 to continue long enough, they would finally lead to the destruction of the 

 solar system, by causing the earth and planets to fall into the sun. But 

 mathematical investigations showed that these changes, after tens of thou- 

 sands of years, will cease, and the orbits will begin to return to their present 

 state, and will never do more than oscillate between narrow limits; these 

 oscillations occupying from fifty thousand to half a million of years. They 

 were illustrated by a chart exhibiting all the changes in the eccentricity of 

 Vesta during the last five hundred thousand years. 



The next question was whether the orbits of Vesta and Hygea could ever 

 have intersected, and this was answered in the negative. The mean distance 

 of Hygea is much greater than that of Vesta, and all but one of the terms in 

 the rigorous expressions for the eccentricity are nearly the same for both 

 planets. The consequence of this is that whenever the orbit of Vesta is 

 elongated in a particular direction, that of Hygea will be elongated in the 

 same direction, and will thus recede from it. Actual calculation showed 

 that they could never have come within much less than thirteen millions of 

 miles of each other. It does not, therefore, seem that Gibers' hypothesis can 

 be true, unless some force of which we have no knowledge has acted on these 

 planets. 



DRY FOGS. 



These phenomena have been lately much discussed by the meteorologists 

 of France. The Abbe Moigno states that these fogs are seen in Belgium and 

 Holland from April to the beginning of June, when the wind is in the north- 

 west, north, or northeast, after the sun has shone for several days. Their 

 appearance coincides generally with a temperature above the mean, but not 

 constantly. They disappear and return again, sometimes after eight days. 

 They do not seem to extend to a great height in the atmosphere, and disap- 

 pear when the wind becomes strong, or when the air is highly charged with 

 humidity. M. Vercruysse, of Courtrai, considers the origin of these fogs 

 may be found in the masses of vegetable matter which cover the shores of 

 Holland and Belgium to a considerable depth. These masses engender 

 grayish-blue vapors, through which the sun appears, especially in the even- 

 ing, of a fiery hue, and which the north wind disperses to a great distance 

 over the country. In 1783, a thick dry fog extended over a great part of 

 Europe. It did not moisten the ground, and appeared like a thick smoke. 

 The sun was so much obscured by it, especially in the morning, that at eight 

 o'clock, when it had well risen, it had to be searched for. During the rest of 

 the day the fog was more elevated. It remained immovable, in spite of the 

 winds and storms which raged above it, and lasted a month, or from June 

 20rh to July 20th. For this phenomenon no satisfactory explanation has 

 over been given. It was, however, referred by some 10 the great earthquakes 



