On a certain summer morning

On a certain summer morning, Hamilton sat at the desk, a stern and sober figure, and Bones, perspiring and rattled, sat on the edge of a chair facing him.
The occasion was a solemn one, for Bones was undergoing his examination in subjects "X" and "Y" for promotion to the rank of Captain. The particular subject under discussion was "Map Reading and Field Sketching," and the inquisition was an oral one.
"Lieutenant Tibbetts," said Hamilton gravely, "you will please define a Base Line."
Bones pushed back the hair straggling over his forehead, and blinked rapidly in an effort of memory.
"A base line, dear old officer?" he repeated. "A base line, dear old Ham----"
"Restrain your endearing terms," said Hamilton, "you won't get any extra marks for 'em."
"A base line?" mused Bones; then, "Whoop! I've got it! God bless your jolly old soul! I thought I'd foozled it. A base line," he said loudly, "is the difference of level between two adjacent contours. How's that, umpire?"
"Wrong," said Hamilton; "you're describing a Vertical Interval."
Bones glared at him.
"Are you sure, dear old chap?" he demanded truculently. "Have a look at the book, jolly old friend, your poor old eyes ain't what they used to be----"
"Lieutenant Tibbetts," said Hamilton in ponderous reproof, "you are behaving very strangely."
"Look here, dear old Ham," wheedled Bones "can't you pretend you asked me what a Vertical Interval was?"
Hamilton reached round to find something to throw, but this was Sanders's study.
"You have a criminal mind, Bones," he said helplessly. "Now get on with it. What are 'Hachures'?"
"Hachures?" said Bones, shutting his eye. "Hachures? Now I know what Hachures are. A lot of people would think they were chickens, but I know ... they're a sort of line ... when you're drawing a hill ... wiggly-waggly lines ... you know the funny things ... a sort of...." Bones made mysterious and erratic gestures in the air, "shading ... water, dear old friend."
"Are you feeling faint?" asked Hamilton, jumping up in alarm.
"No, silly ass ... shadings ... direction of water--am I right, sir?"
"Not being a thought-reader I can't visualize your disordered mind," said Hamilton, "but Hachures are the conventional method of representing hill features by shading in short vertical lines to indicate the slope and the water flow. I gather that you have a hazy idea of what the answer should be."
"I thank you, dear old sir, for that generous tribute to my grasp of military science," said Bones. "An' now proceed to the next torture--which will you have, sir, rack or thumbscrew?--oh, thank you, Horace, I'll have a glass of boiling oil."
"Shut up talking to yourself," growled Hamilton, "and tell me what is meant by 'Orienting a Map'?"
"Turning it to the east," said Bones promptly. "Next, sir."
"What is meant by 'Orienting a Map'?" asked Hamilton patiently.
"I've told you once," said Bones defiantly.
"Orienting a Map," said Hamilton, "as I have explained to you a thousand times, means setting your map or plane-table so that the north line lies north."