Shakespeare's Sonnets: The other two, slight air and purging fire

The other two, slight air and purging fire,Are both with thee, where ever I abide;The first my thought, the other my desire,These present-absent with swift motion slide.For when these quicker elements are goneIn tender embassy of love to thee,My life, being made of four, with two aloneSinks down to death, oppress't with melancholy.Until life's composition be recuredBy those swift messengers return'd from thee,Who ev'n but now come back again assuredOf their fair health, recounting it to me. This told, I joy, but then no longer glad, I send them back again and straight grow sad.