36 Questions for Solution relating to Meteorology^ 



been solved in a satisfactory manner. The study of this phe- 

 nomenon is chiefly reserved, by the very nature of things, to 

 observers placed in the equinoctial regions. They alone can 

 decide whether Dominique Cassini has sufficiently guarded 

 against the causes of error to which an observer is exposed in 

 our variable climates, arH whether he has sufficiently taken into 

 account the purity of the air, when he announced in his work 

 that the zodiacal light is constantly brighter in the evening than 

 in the morning ; that in the course of a few days its length may 

 vary from 60° to 100° ; that these variations are connected with 

 the appearance of solar spots, in such a manner that there must 

 have been, for instance, a direct dependence and not merely a 

 fortuitous coincidence, between the weakness of the zodiacal 

 light in 1688, and the absence of every kind of spot or foecula on 

 the solar disc in that same year. 



It appears to us, therefore, that the Academy ought to desire 

 the officers of the Bonite^ during the whole time they remain 

 between the tropics, and when the moon does not lighten the 

 horizon, to be on the watch, either after sunset or before sun-ris- 

 ing, and take note of the constellations which the zodiacal light 

 traverses, of the star which indicates its point, and of the angu- 

 lar breadth of the phenomenon near the horizon, at a deter- 

 mined height. It is almost superfluous to add, that an account 

 must be kept of the hours when the observations were made. 

 The calculation of the results may be delayed without any 

 inconvenience till the period of returning home. 



We are not ignorant, as has been already hinted, that some 

 very able observers regard these statements of Dominique Cas- 

 sini as little deserving of confidence. They are unwilling to 

 admit that sensible physical changes could operate simultaneously 

 in such an immense extent as the zodiacal light embraces. In 

 their opinion, the variations in intensity and length noticed by 

 this great astronomer, were not real, and they think that we 

 need go no farther for an explanation of them, than the inter- 

 missions of the atmospheric transparency. 



It would not, perhaps, be impossible to find proof in the ob- 

 servations of Fatio, compared with those of Cassini, that atmo- 

 -spheric variations are insufficient to explain the phenomena de- 

 scribed by the Parisian astronomer. With respect to the objec- 



