Notwithstanding these seeming inconsistencies with the law of universal gravitation, to which the explanation of the tides is referred, the correspondence of the tides to the motions of the sun and moon, in obedience to the law of attraction, is in general such as to warrant the application of that law to them, while in a great majority of the cases which appear to be exceptions to the operation of that law, local causes and impediments have been discovered, which modified or overruled the uniform operation of the law of gravitation. Thus it does not disprove the reality of the existence of a force which carries bodies near the surface of the earth towards its centre, that we see them sometimes compelled, by the operation of local causes, to move in the opposite direction. A ball shot from a cannon is still subject to the law of gravitation, although, for a certain time, in obedience to the impulse given it, it may proceed in a line contrary to that in which gravity alone would carry it. The fact that water may be made to run up hill does not disprove the fact that it usually runs down hill by the force of gravity, or that it is still subject to this force, although, from the action of modifying or superior forces, it may be proceeding in a direction contrary to that given by gravity. Indeed, those who have studied the doctrine of the tides most profoundly consider them as affording a striking and palpable exhibition of the truth of the doctrine of universal gravitation. FOOTNOTE: [11] The exact longitude of the City Hall, in the city of New York, is 4h. 56m. 33.5s. LETTER XX. PLANETS.--MERCURY AND VENUS. "First, Mercury, amidst full tides of light, Rolls next the sun, through his small circle bright; Our earth would blaze beneath so fierce a ray, And all its marble mountains melt away. Fair Venus next fulfils her larger round, With softer beams, and milder glory crowned; Friend to mankind, she glitters from afar, Now the bright evening, now the morning, star."--_Baker._