wt 
- 
152 Remarks on Dr. Enfield’s Institutes 
among those which compose the chapter, is borrowed iroin 
‘Rutherforth. ma . 
Prop 168. Schol. This method of finding the direction 
of gravity includes only the effect of the centrifugal force. 
Including the joint effect of rotation and of figure, the di- 
rection is manifestly that of a perpendicular to the tangent 
_ plane of the earth’s surface, or of a normal to the elliptical 
curve of the meridian passing through the given place. — 
Prop. 173. In the concluding paragraph of the demon- 
stration, the relative forces of the sun and moon to raise 
ides are erroneously stated. The real forces are directly 
as the masses, and inversely as the cubes of the distances. 
The concluding scholium of the Astronomy consists of 
extracts from a paper of Dr. Herschel’s in the Philos. 
Trans. for 1795. These extracts are so unskilfully made, 
nal. But in the original itself, high as is the estimation in 
which the author is justly held as an observer, we must be 
points of analogy between the sun and the planets, and 
thus increasing the presumption that the former is mhabit- 
ed, he endeavours to shew that both primaries and seconda- 
ries shine in some measure by-their own light. The partial 
illumination of the moon, for example, during a total eclipse, 
cannot be entirely ascribed to the light which may reach it 
from the earth’s atmosphere ;—“ because, in some cases, 
the focus of the sun’s rays refracted by the earth’s atmos- 
phere must be many thousand miles. beyond the moon.” 
r. Herschel assumes as the basis of this calculation that 
the rays of the sun are bent by the atmosphere at only an 
angle of 31’. He seems to have inadvertently neglected | 
the circumstance that the rays undergo a second equal re- 
fraction in passing out of the atmosphere. In consequence 
of this, the real inflection is 62’, (or rather 66’, taking 33° 
as the mean horizontal refraction,) so that the rane. 
sun’s rays as refracted by the earth’s atmosphere can nevet 
in fact be so distant as the moon. An observer stationed at 
‘the moon, even during a central eclipse of the sun, would 
see a luminous ring encircling the earth. The light thus 
thrown: upon the moon’s disc is amply sufficient to explain 
