In the course of his lecture, he said:—Sometimes it was easy to find the way, but at other times it was extraordinarily difficult, and he would describe certain methods they had for getting over those difficulties; they were not perfect nor were the instruments used perfect, so he hoped that someone there might hit upon improvements arising out of what he was going to say, and, if so, he hoped they would write and tell him all about it.

Suppose we were in the middle of Hampstead Heath on a very dark night and that there were no lamps nor lanterns that one could recognise and we wanted to find our way. One would, of course, suggest that a compass would first be necessary. A night marching compass with a luminous dial was the next step. With these we could find how far one had gone in a certain direction.