Mercury is the planet closest to the Sun. Till not so long ago astronomers thought that Mercury always showed the same 'face' to the Sun but now they have found that Mercury turns in a way so it rotates slowly. A Mercury day lasts 2/3rds of a Mercury year (88 earth days).
Mercury is the fastest mover of all planets, 48 km/s. It has phases as do Venus and the Moon. These phases dominate - just before or after a conjunction Mercury is the most visible.
Most of the year we cannot see Mercury. Sometimes it is visible at dawn or dusk, but only weakly. Once in a while it will pass over the Sun and then one can see it as a very small dark ball. For the rest it plays bo-peep around the great light of the Sun.
Mercury was the god of healing, of business deals and of thieves. In all of these professions one has to make quick, to the point judgments and quick use of opportunities. Mercury is also a fast mover and as hard to grab hold of as quicksilver. He moves things around, not always with permission (as with thieves). He will bring movement into stalemate situations (as in healing). He changes forms which have become stabilized - movement without form is his ideal. He feels the cosmic rhythms and adapts to them instinctively. If he were conscious of this he would probably reject it for tying him down.
Mercury is a fast thinker, rational and on the look-out for fast deals in his favor. His thinking is short term, in the present. He is quick to contradict and can think up plausible sounding arguments quicker than anyone can think up answers to them. Mercury has thousands of unrelated facts at his fingertips. He can combine them at will and dazzle you with them. If he sees that you know more about the subject than he does, he will just go off in another direction with a joke. Humor is important; he is the only planet who really uses humor. The witty repartee. His function is to shake others out of their self-satisfied routines and traditions.
As has been said Mercury is the court jester to Jupiter. Jupiter is ponderous and Mercury loves to prick holes in anyone's convictions and Jupiter is the only one who appreciates this. Saturn cannot keep up with Mercury so Mercury finds Saturn very dull indeed. Mars is often irritated by Mercury, though Mercury appreciates the practicality of Mars in getting things done. In groups, when the situation has become tense or (in Mercury's opinion) too serious, he will make a joke or a ridiculous remark which loosens the tension by laughter. The metal mercury dissolves all other metals.
In feeling Mercury is superficial. He knows everybody but has no real friends. He plays too many roles for that. As the planet Mercury revolves so slowly (a day takes two thirds of a Mercury year) that it shows the same face to the Sun for a long time, so a person with Mercury qualities looks for someone to look up to, to show his 'real' self to, someone who will accept him as he is (or as he wants to be). This person is often a Jupiter and Mercury will try to be more or less consistent with him and try not to practice his wheeling and dealing on him (though usually in the end he can't keep this up).
Mercury does not have will power for long-term projects or thinking things through. On a short-term base he knows what he wants, which role he has to play to get it and he sees this through. His solutions to things are never 'final' solutions. He just keeps moving (things, ideas, himself) till somebody else picks out one of his ideas and stays with it. Mercury will then naturally ask for a commission (and usually get it).
Quarrels with Mercury are short, sharp and cutting. They usually end with a joke from Mercury. With two Mercury people this is the end of the quarrel - they both laugh and forget. Mars will accept this if they both go back to work. With Venus it depends. Venus can laugh about emotions because they are shared; Mercury makes intellectual jokes, for instance playing on words. But Venus can laugh at the antics of a child and in this way she can finish the quarrel with a laugh with Mercury. Saturn will never get over the quarrel, because Mercury will refuse to talk it out in depth at a later time.
The body parts ruled by Mercury are eyes, lungs and the mucous membranes. The eyes that register everything. The lungs that function as an intermediary between the cosmos and the individual. The mucous membranes are a continuation of the skin in the form of a coating of internal cavities. There is a great variety because every coating has its own speciality.
Schooling task for Mercury: Mercury instinctively feels the cosmic rhythms; he should make them conscious and work with them. He should become more conscious of his inner self and motivate himself to become more helpful without looking for material rewards.
Example: In the example of the dike burst and the flood Mercury will be there taking photographs (to sell to the newspapers), transporting people to temporary housing 'that only he knows of' and at a price of course.