How does a dog know what direction to go with a track?

This comes from a book I have Called "Meet Mr. Grizzly" by Montague Stevens (1943)

He used hounds to hunt grizzlys and owned a ranch out west where he often had his hired hands lay human scent trails for his hounds to learn from. Which is why he says the trail of a man.

"While there is no yard-stick by which to measure the keenness of a hounds scent, there is, however, a way in which to get an approximate idea. When a hound approaches the trail of a man at right angles, and scents the trail in the air, how does he know weather the man went to the right or two the left? Usually he takes the correct end of the trail; that is, the way the man went, the other way being called his back trail. But sometimes, due probably to the wind or some other atmospheric condition, the hound is not sure which way to go and he will run some 20 yards in each direction alternately several times, untill he has fully made up his mind which is the right end of the trail to take. The question therefore arises: how much difference is there in the strength of the scent twenty yards one side or the other of a given point. That difference is the measure of the keenness of a hound's scent"

This guy also beleived that often times a dog will run a trail by scenting the the air on one side or the other of the actual trail which is where the scent has disperced to, often due to the wind. Ive seen dogs run a trail 10 or so yards off of where the hog actually ran by a min or so before. Chances are the dog was on the down wind side of where the hog ran which is where the scent would be the strongest

Waylon