I have a 1987 volvo wagon 740 model. I need to find a picture of its undercarriage so that I can determine what the long metal tube like piece is that has a hole in it that is leaking water.

Have you tried a repair manual? Try contacting Volvo themselves, I'm pretty sure they have archives of pictures.

When do you notice it leaking?
Can you get us a picture where you believe the leak is coming from?