"Well," explained Humphrey, "the first thing to do in a criminal case is to take a good survey of the problem and the ground it covers—just as you do when you build a bridge, a dam, or open up a mine. The higher type of criminal investigator usually falls back upon his study and knowledge of criminology, which is a broader and more scientific development of the ordinary _facts_ with which the city detective starts to work. What I am going to suggest to you is a new branch of criminology. For want of a better name at this time we’ll call it Engineering-Criminology. I am not an engineer myself, and what I know about surveying and triangulation could be put into a thimble, but I think I know enough to give you an idea of what I mean. "As I said before, the detectives are wallowing around in the mire of the lower West Side—they are in the valley, so to speak. Now, if you, as an engineer, were about to survey a certain unknown and inaccessible territory, you’d go up on the nearest high hill and pick out two other prominent points in the landscape, so as to form a triangle. Then you’d take sights, or whatever you would call it, from one point to another. A little figuring would give you the exact distance from one point to another, and a lot of information about the lay of the land in between. Am I not right?" "You’ve put it very roughly, but I think I can see what you are driving at," returned Forrester. "Continue the idea a little further, Mr. Forrester," went on Humphrey. "If you were making a mine survey you would first lay out your boundaries, tunnels and so on, on the surface, wouldn’t you?" Forrester nodded. "Then you would carry those lines below the surface. In other words, the lines you had laid out _on_ the surface would be a guide to you when you got _below_ it." Again Forrester nodded. "All right," said Humphrey. "I think you’ve got my idea about this case, and what I mean by solving it by triangulation. You will take the people, and the events which have occurred, and use them as your prominent landmarks; that is, points for your triangles. You will then study what lies between those landmarks, and also what lies under the surface. By that means I think you will eventually discover some clues that will be worth while.