224 • DIRECTIONS FOR OBSERVING 



sky, especially clouds chased by the wind. The fact is certain. I 

 have recognized its truth in thousands of circumstances, audi am not 

 aware of having detected a single exception. In view of this, I do 

 not say that observations made on stars in the vicinity of clouds are 

 to be rejected, for these observations may also have their importance, 

 but it is necessary that regard should be had to this circumstance 

 which sensibly modifies the results. 



As the light of the moon interferes with observations, those made 

 at full moon must necessarily be less exact than those made on moon- 

 less nights. 



4. Comparison of ohservations made by different jjerson-s. — Here is 

 the delicate point. What has been said may serve for the study of 

 the phenomenon of scintillation when a single person is charged with 

 making all the observations. But when there are several, how shall 

 we know that the scintillation which one observer designates by 2.5 

 is equal to that which another designates by 2.5? 



This uniformity I believe it impossible to realize; and, unless the 

 observers were formed in the school of more experienced observers, 

 I am certain that it is not to be attained. Yet I shall proceed to 

 show that there is a mode of recognizing whether, in absolute value, 

 the scintillation is stronger at one station than at another station. 



At Merges, on nights of strong scintillation the stars in the zenith 

 have a very distinct scintillation. During nights of average scintil- 

 lation, the scintillation of stars at a very high elevation is weak, 

 though always appreciable. But, on nights when scintillation is weak, 

 the stars near the zenith have none at all. And the weaker the scin- 

 tillation duiing an evening, the more extended is the spherical canopy 

 of which the zenith is always the centre, and which comprehends the 

 stars whose scintillation is inappreciable. 



I have sometimes observed that at a time of very weak scintillation 

 the stars ceased to scintillate when they had ascended 42° above the 

 horizon, but I have never seen scintillation entirely cease in stars at 

 a lower elevation; and yet, according to Arago, this sometimes hap- 

 pens. He cites, among others, the observations of M. de Humboldt, 

 who says: On the banks of the Oronoco no scintillation can be dis- 

 tinguished in the stars, even at 4*^ to 5° above the horizon. Le 

 Gentil asserted, that at Pondicherry, in the months of January and 

 February, the stars have no scintillation. Beauchamp writes to 

 Lalande that at Bagdad the stars, when they had ascended 45° above 

 the horizon, no longer scintillated. 



Garcin announced in 1743 that at Bender Abassi, on the shores of 

 the Persian gulf, in spring, summer, 'and autumn, the stars showed 

 no scintillation; it was in winter, only, that a slight one could be dis- 

 covered. According to Humboldt, scintillation is, in general, not 

 perceptible at Cumana when the stars are at an elevation of 25°, &c. 

 Assuredly, I have never witnessed at Morges, during six years, so 

 weak a scintillation, but in thus discriminating the heights at which 

 the stars cease to scintillate, the observations which I have made are 

 susceptible of being compared with those of Le Gentil and Humboldt. 

 Hence I particularly invite the attention of observers to this point, 



