LECTURE VI.

THE PLANET MERCURY.

“ First Mercury completes his transient year,

Glowing refulgent with reflected glare.”— Cliatterton.

“ Neque a Sole longius unquam unins signi intervallo discedit, turn antevertens,turn subsequens.”— Cicero.

In passing on to discuss the various planets individually, itwill, we think, he most convenient to take them in orderaccording to their distances from the Sun, beginning with thenearest. In attempting to do so, we must, however, at onceface the question:—Do we know which is the nearest? Wehave already mentioned that the proximity of Mercury to theSun makes it often impossible to observe it; may we not thenask, whether, in spite of all our recent progress, in spite ofall the advance that has been made since the early days of theChaldasan astronomy, there may not he planets between the■Sun and Mercury which we have as yet failed to detect? Wenotice in Bode’s so-called law, that the first term, which istaken to represent the distance of Mercury , is an empiricalone; and for this, as well as for other reasons, we feel that wehave no good grounds for affirming that there is any d prioriimprobability against the existence of such planets.

On the other hand, Le Verrier , than whom no greater autho-rity could be quoted, has made elaborate calculations of certainpeculiarities in the movement of Mercury , which indicate thata very considerable amount of matter lying between it and theSun must constantly exercise an attractive effect upon thatplanet. He has said that a body of about -|rds of the weight ofMercury , revolving at one-half of its distance from the Sun, wouldproduce the required effect; or, that a larger amount of matter,