the Scintillation of the Stars. 219 
sometimes as 4, and sometimes as 8, &c., and gives the follow- 
ing reasons for so doing :—“ This scale that I have employed 
is an imaginary division for the estimation of a phenomenon for 
which we can have no real measure. I suppose, then, that the 
highest degree is 10, the lowest 1; and I shall endeavour to 
determine the intermediate degrees either by the intensity of the 
sensation produced, or by other circumstances connected with 
the phenomenon. This appears to me to present ideas with 
more precision than by simply qualifying the different degrees 
by the vague terms of strong, middling, and feeble. Thus I 
would estimate at 8 degrees the fog of July 3, 1783.” (De Saus- 
sure, Travels in the Alps: Third Journey, chap. 2.) 
I have explained the scale I employ ; other observers can form 
their own, according to their ideas of what is best. The num- 
bers can be changed at will, so long as the degrees remain prac- 
tically the same. Thus, my brother Mark, who began his ob- 
servations about a year ago, has adopted a scale much lower than 
mine; but we always agree as to the amount of scintillation on 
any given evening, and also as to whether one star scintillates 
more than another. These are the most important poits to 
decide on; for, as all the observations cannot be taken by the 
same person, it would often be difficult to ascertain if the scin- 
tillation is of the same intensity at Cairo, New Archangel, on 
the Peak of Teneriffe, or at Morges. 
Within certain limits this difficulty can be obviated, as I will 
endeavour to show later; but in any case, by following the 
method of instruction I have indicated, it will be possible to 
know how much the scintillation varies from one night to an- 
other, and whether the amount appears affected or is influenced 
by any meteorological perturbations. 
It is of course needless to add, that the height of the star ob- 
served must be known and noted. It is not necessary, however, 
to take a direct observation of the height of the star in question ; 
it is much easier to caleulate this from the time of the night; 
and, to abridge these calculations, a Table can be drawn up indi- 
cating what is the exact height of the stars to be observed in the 
different sidereal hours, according to the latitude of the obser- 
ver’s position. The Table I have gives the degrees of height for 
Morges, calculated at intervals of half an hour. And this Table 
is sufficient ; for in the space of half an hour one can interpolate 
with all the exactitude necessary, as in this case it is only need- 
ful to know approximatively, within a degrec or half a degree, the 
height of the stars. 
2nd. Mode of comparing observations. 
It has been proved that, all other circumstances being equal, 
the scintillation of the stars decreases in proportion as they ap- 
