William Shakespeare: The other two, slight air, and purging fire

The other two, slight air, and purging fire
Are both with thee, wherever I abide;
The first my thought, the other my desire,
These present-absent with swift motion slide.
For when these quicker elements are gone
In tender embassy of love to thee,
My life, being made of four, with two alone
Sinks down to death, oppress'd with melancholy;
Until life's composition be recur'd
By those swift messengers return'd from thee,
Who even but now come back again, assur'd,
Of thy fair health, recounting it to me:
This told, I joy; but then no longer glad,
I send them back again, and straight grow sad.