f>64 



several successive years. Has the light appeared propor- 

 tionately weakened in such years in the equinoctial zone also ? 

 The investigation must not, however, be restricted to the 

 jtatement of the configuration according to the distance from 

 known stars or direct measurements. The intensity of the 

 light, its uniformity or probable intermittence (darting and 

 flashing), its analysis by the polariscope, should be especially 

 investigated. Arago (Annuaire pour 1836, p. 289) has 

 already pointed out that the comparative observation <*f 

 Dominique Cassini, would perhaps clearly prove " que la 

 supposition des intermittences de la diaphanite atmosphe- 

 rique ne saurait suffire a 1* explication des variations sigimlees 

 par cet astronome." "That the supposition of intermittent 

 variations in the diaphanity of the atmosphere, would not 

 suffice for the explanation of the changes indicated by that 

 astronomer." 



Immediately after the observations of this great astronomer 

 at Paris, and of his friend Fatio de Duillier, an inclination to 

 similar labours showed itself in Indian travellers (Father 

 Noel, de Beze and Duhalde) ; but isolated reports (for the 

 greater part only describing the gratification experienced at 

 the unusual prospect) are not available for the sound discus- 

 sion of the causes of the variability. It is not by rapid travels 

 or so-called voyages round the world, as the labours of the active 

 Horner have recently shown (ZacJi, Monatl. Corresp. bd. x. 

 pp. 337-340), that the deserved object is to be obtained. It 

 is only by a permanent stay of several years in some tropical 

 country, that the problem of variable configuration and lumi- 

 nous intensity can be solved. Therefore, the most is to be 

 expected for the subject which now occupies us, as well as for 

 the entire science of meteorology, from the ultimate diffusion 

 of scientific culture throughout the equinoctial world, the 

 former Spanish America, where large popolous towns, Cuzco, 

 La Paz, Potosi, are situated between 10,700 and 12,500 



