vinced me that such a device would not be practicable on a tube of the great length this one will have. I have, however, found a plan, on much the same principle, that promises to give far better results than the other."
"I thought it was only pretty women who were allowed to chenge their minds," said Mr. Curtis, smiling.
"Pretty women and intelligent men," retorted Dr. Giles. "A man who has n't sense enough to change his mind when he has a good reason for doing so, is little better than a fool. A truly scientific man is always ready to give up one idea for a better one."
"Well, then, tell me your new idea, I beg, that I may see if it is in keeping with the rest of your absurd plan."
"I have n't time to go into much detail," said the doctor, "but I can briefly explain it to you in outline."
"Please do, and make your explanation as simple as you can. I always like explanations that are clear and easy to understand."
"All right. Now, in the present case, the problem is simply this: The heat has been steadily increasing the farther down we get into the in-
