222 Prof. Dufour : Instructions for the better observation of 
boldt, who says that on the borders of the Orinoco no seintil- 
lation can be observed, not even at 4° or 5° above the horizon. 
Le Gentil asserts that at Pondicherry, during the months of 
January and February, the stars do not scintillate at all. Beau- 
champ wrote to Lalande, that at Bagdad the stars ceased to 
scintillate as soon as they arrived at 45° above the horizon. 
Garcin asserted, in 1743, that at Bender Abassi, on the 
borders of the Persian Gulf, in the spring, summer, and autumn 
the stars did not scintillate ; it was only in winter that a slight 
scintillation was perceptible. 
According to M. de Humboldt, at Cumana in general the 
scintillation is no longer sensible when the stars are more than 
25° above the horizon, &c. Most assuredly I have never seen 
at Morges, during the past six years, so small an amount of 
scintillation; but it is by specifying the various heights at which 
stars cease to scintillate that my observations are susceptible of 
being compared with those of M. de Humboldt and Le Gentil. 
I therefore call the attention of observers specially to this point, 
as probably the best manner of comparing the calculations 
obtained in various parts of the globe. 
The scintillation of stars of the first magnitude is perfectly 
appreciable by the naked eye; that of the secondary ones less 
so; and so on, until for the smallest and least brilliant stars it 
becomes quite inappreciable: but this limit varies from one 
night to another, according as the general scintillation is strong 
or feeble. This fact may also serve as a means of comparing 
the degrees of scintillation. 
The magnitude of the stars that appear to scintillate must be 
noted ; and as we have seen that the height of the stars above 
the horizon exercises a great influence on the intensity of the 
phenomenon, it will be necessary to name some of the stars, 
and to indicate, besides, their height above the horizon, or at 
least the time at which the observation was taken ; but I attach 
less importance to this method of comparison than to the pre- 
ceding one, because the purity of the atmosphere, the sight of 
the observer, and the various degrees in the brillianey of the 
stars, all exert more or less influence on the results obtained. 
5th. Variable Stars. 
Of all the stars I have observed, « of Orion is the one of which 
the scintillation appears to me to be the most irregular; but it 
is well known that the brilliancy of this star is not always the 
same: and with reference to the variable stars, all that is known 
of them is the duration of their periods ; and consequently their 
scintillation is also an interesting phenomenon to study. It would 
be, above all, interesting in the case of the star 7 of Argo, whose 
