By all my library training in detective work the next move obviously was to find what color of cigarette ashes the Turk smoked.  Instead I blundered upon the absurdly simple notion of trying to locate the negro of given length and proportions.  The real “mosquito man”—­one of that dark band that spends its Zone years with a wire hook and a screened bucket gathering evidence against the defenseless mosquito for the sanitary department to gloat over—­was found not to fit the model even in hue.  Moreover, “mosquito men” are not accustomed to carry their devotion to duty to the point of crawling under trunks in their quest.

For a few days following, the hunt led me through all Gatun and vicinity.  Now I found myself racing across the narrow plank bridges above the yawning gulf of the locks, with far below tiny men and toy trains, now in and out among the cathedral-like flying buttresses, under the giant arches past staring signs of “Danger!” on every hand—­as if one could not plainly hear its presence without the posting.  I descended to the very floor of the locks, far below the earth, and tramped the long half-mile of the three flights between soaring concrete walls.  Above me rose the great steel gates, standing ajar and giving one the impression of an opening in the Great Wall of China or of a sky-scraper about to be swung lightly aside.  On them resounded the roar of the compressed-air riveters and all the way up the sheer faces, growing smaller and smaller as they neared the sky, were McClintic-Marshall men driving into place red-hot rivets, thrown at them viciously by negroes at the forges and glaring like comets’ tails against the twilight void.

The chase sent me more than once stumbling away across rock-tumbled Gatun dam that squats its vast bulk where for long centuries, eighty-five feet below, was the village of Old Gatun with its proud church and its checkered history, where Morgan and Peruvian viceroys and “Forty-niners” were wont to pause from their arduous journeyings.  They call it a dam.  It is rather a range of hills, a part and portion of the highlands that, east and west, enclose the valley of the Chagres, its summit resembling the terminal yards of some great city.  There was one day when I sought a negro brakeman attached to a given locomotive.  I climbed to a yard-master’s tower above the Spillway and the yard-master, taking up his powerful field-glasses, swept the horizon, or rather the dam, and discovered the engine for me as a mariner discovers an island at sea.

“Er—­would you be kind enough to tell us where we can find this Gatun dam we’ve heard so much about?” asked a party of four tourists, half and half as to sex, who had been wandering about on it for an hour or so with puzzled expressions of countenance.  They addressed themselves to a busy civil engineer in leather leggings and rolled up shirt sleeves.

“I’m sorry I haven’t time to use the instrument,” replied the engineer over his shoulder, while he wig-wagged his orders to his negro helpers scattered over the landscape, “but as nearly as I can tell with the naked eye, you are now standing in the exact center of it.”