This sentence is from the first chapter of The Strange Case of Dr. Jekyll and Mr. Hyde:
It was reported by those who encountered them in their Sunday walks, that they said nothing, looked singularly dull, and would hail with obvious relief the appearance of a friend.
I'm not sure what the word hail means in this context.
Best Answer
Picture this: You are waiting at the train station for a friend to show up. The train is about to depart and you are anxious not to miss it. There are people all around - you look everywhere but cannot see your friend. Suddenly, your friend appears, so you jump, wave and smile at them (this is hailing them - hail defined by Dictionary.com as to cheer, salute or greet) , relieved to see them suddenly appear just in time to catch the train. The word "obvious" is used because you clearly show relief with your body language. "Phew, that was close! I am very glad to see you arrived in time!"
That's the sort of thing it is portraying.