a Theory of the Earth . 461 
only a resemblance of properties, or in their manner of 
action ? 
10. Is it well ascertained, as M. Van Swinden thinks, 
that the aurora borealis acts on the magnetic needle ; and 
can any idea be formed of the mode of this action ? 
1 1 . The same question in regard to the zodiacal light, 
12. In general, the theory of the magnet is still so far 
from perfection, even in that part which depends merely 
on observation, that it is much to be wished that observers 
and observations were multiplied, especially in what con- 
cerns the inclination of the needle. In regard to the de- 
clination and its variations, M. Van Swinden has given 
a noble example of correctness, and of constancy in obser- 
vations, and in the art of classing and comparing the 
results. It would be of great benefit if this example were 
followed in different situations and climates. It would be 
highly interesting, for instance, to determine, with preci- 
sion, the zones of the earth where the declination is no- 
thing, and where changes take place, and the same for the 
inclination. 
CHAP. XXII, 
Errors to be avoided in Observations respecting Geology . 
1. THERE are some errors into which people may 
readily fall when they have not had long experience in 
any given kind of observation, and against which it is of 
importance to put beginners at least on their guard. 
2. One may be readily deceived in regard to the rela- 
tive distances of remote objects. All the stars and 
planets appear to be at an equal distance from us. Distant 
mountains all appear to be in the same plain. Thus 
those which are situated very far behind the rest, seem 
to form one body with them ; so that people believe they 
see continued and uninterrupted chains when there are 
