For a time, we sat in silence. Our cab had reached the beginnings of the City and I was gazing out of the window, my fingers drumming idly on the half–lowered pane, which was already befogged with moisture, when my thoughts were recalled by a sharp ejaculation from my companion. He was staring fixedly over my shoulder.

"The glass," he muttered.

Over the clouded surface there now lay an intricate tracery of whorls and lines where my finger had wandered aimlessly.

Holmes clapped his hand to his brow and, throwing open the other window, he shouted an order to the cabby.