Sidereal Astronomy. 
1876.] 
75 
parably heavier than the earth on which we are, heavier 
even than our colossal sun. 
The power which we have of weighing a star is, 
unquestionably, one of the most surprising results of the 
progress of science, one of those which leaves the greatest 
doubt in the mind of persons unaccustomed to the principles 
of celestial mechanism. To weigh a star is a work even more 
extraordinary than to measure its distance, and neither 
Copernicus, Galileo, Kepler, nor Newton certainly ever 
dreamt the day was coming when their successors 
would be capable, by the application of their immortal dis- 
coveries, to determine the mass of a star lost in the depths 
of celestial space. Therefore, we think we ought to com- 
plete this summary statement of our aCtual knowledge of 
the double stars by indicating the results arrived at on the 
magnitude and masses of these systems, and in giving an 
idea of the method employed to obtain them. 
The mass of a star is calculated by the energy of the 
adtion which it exercises around itself. If the earth was ten 
times heavier than it is, preserving always the same volume, 
it would attract bodies towards its surface ten times more 
strongly than it does now,' and an objeCt which in falling 
travels over 16 feet in the first second of falling would travel 
160 feet. If the Earth, whilst preserving its volume, had the 
mass of the sun, it would attraCt bodies324, 000 times stronger; 
and an objeCt which now weighs 1 kilog. would weigh 
324,000. A man of the average weight of 70 kilog., would 
weigh 22 millions ! We can measure the weight of a star 
by the intensity of the attraction at its surface. Reduced 
to its simplest expression in its application to the fall of 
bodies, this attraction would be difficult to verify; but we 
can determine it by the speed of a satellite revolving round 
the star of which we wish to know the mass. 
For example, the attraction of the earth has the power of 
curving the right line, which would be followed by the moon 
in space if it did not follow this attraction ; and it curves 
by attracting it in such a manner that it makes a circum- 
ference travelled over in 27 days 7 hours 43 minutes. If 
the mass or energy of the earth were to augment, the velocity 
of the moon in its orbit would increase also : if it were to 
diminish, the contrary effeCt would be produced. The 
attraction varies in direCt proportion with the masses. The 
quickness of the moon’s movement round the earth comes 
from the aCtual force of the earth. The earth is the hand 
which makes the moon turn in opposition. If the earth had 
more power, more energy than it has, it would make the 
