This expression, made by Marlow in Oliver Goldsmith's play She Stoops to Conquer, is a reaction to Tony, who is about to play a really bad prank on him.,
Tony is going to confuse Marlow by telling him the directions to go find the closest inn (public house) in town. This is no inn; in fact, it is the Hardcastle estate where Marlow is to show up to ask the hand of Miss Kate Hardcastle in marriage. However, when Tony gives Marlow the directions to go to the Hardcastles he and Hastings complot to confuse Marlow even further.
It's a damned long, dark, boggy, dirty, dangerous way. Stingo, tell the gentlemen the way to Mr. Hardcastle's ; (winking upon the Landlord,) Mr. Hard- castle's,' of Quagmire Marsh — you under- stand me.
Zounds man! We could as soon find out the longitude
comes as a result of the ridiculous and over-complicated directions that Tony gives Marlow. Tony tells him everything he can think of, from going to the bottom of a hill, to finding the track of a wheel, and then right on the farmer's barn. There is almost no ending to the directions.
Hence Marlow responds by saying "Zounds, man!" which is an archaic English utterance that indicates indignation or frustration. Then he sarcastically adds that, considering the excessive amount of directions, he and his companion would find it easier to measure the entire route rather than follow each and every step that Tony suggests. In archaic, longitude refers to a very long distance. The word is also used in terms of "celestial longitude" which also implies the measurement in distance from one place to another. Therefore, what Marlow is saying is that essentially it is easier to find the hardest thing, which is the entire longitudinal measurement of the route, rather than do what Tony says.