I'm a bit worried about a couple friends, and somehow this seems to fit my mood today.
John Donne is one of my favorite early modern poets and essayists. He seems to get at the complexity and difficulties of faith and sexuality more fully than other poets for me (though Herbert's not half bad at the faith part). Unfortunately, he's also become the subject of so many bad puns that it's not even funny anymore, so I'm resisting!
A quick note: a "compass" is the tool you probably used when you were a kid in geometry class to draw circles. In the illustration on the Wikipedia link, the one on the right is basically what you should picture when you get to the compass conceit. The idea is that it's got two "legs," one of which has a pin for a "foot," and stays put at the center of the circle or arc you're drawing, while the other has a lead or other marking tool, and produces a circle or arc. You can draw the second leg up to the first, which makes the instrument "taller" and "erect."
The compass here provides a nice example of a metaphysical conceit, which is basically a drawn out metaphor (a metaphor is a replacement of one term by another, figurative one, a process which invites the reader to think through the ways the terms are alike and different) where the point(s) of similarity tend to stretch the imagination. Basically, it asks the reader to think about the ways in which a compass is like a pair of lovers, and then to realize how difficult that comparison is by thinking about how unlike the things are. (If you like this sort of thing as I do, you should check out other metaphysical poets!)
A VALEDICTION FORBIDDING MOURNING
by John Donne
AS virtuous men pass mildly away,
And whisper to their souls to go,
Whilst some of their sad friends do say,
"Now his breath goes," and some say, "No."
So let us melt, and make no noise, 
No tear-floods, nor sigh-tempests move;
'Twere profanation of our joys
To tell the laity our love.
Moving of th' earth brings harms and fears;
Men reckon what it did, and meant; 
But trepidation of the spheres,
Though greater far, is innocent.
Dull sublunary lovers' love
—Whose soul is sense—cannot admit
Of absence, 'cause it doth remove 
The thing which elemented it.
But we by a love so much refined,
That ourselves know not what it is,
Inter-assurèd of the mind,
Care less, eyes, lips and hands to miss. 
Our two souls therefore, which are one,
Though I must go, endure not yet
A breach, but an expansion,
Like gold to aery thinness beat.
If they be two, they are two so 
As stiff twin compasses are two;
Thy soul, the fix'd foot, makes no show
To move, but doth, if th' other do.
And though it in the centre sit,
Yet, when the other far doth roam, 
It leans, and hearkens after it,
And grows erect, as that comes home.
Such wilt thou be to me, who must,
Like th' other foot, obliquely run;
Thy firmness makes my circle just, 
And makes me end where I begun.