lie in such remote parts of the Earth that we cannot afford to send many astronomers there; but there are few parts of the Earth so remote that the help of British soldiers or sailors cannot be obtained; and with such help even a single astronomer may arrange his work so as to secure a number of valuable records.
To bring home to you the exciting nature of work of this kind, we will pretend to have an eclipse in this room. Here is the astronomer come to choose his site for observation (one of the "juvenile audience," previously instructed, here selected a site on the lecture room floor); he calls in the aid of His Majesty's forces (here several boy scouts entered the room) and directs them to erect his piers and instruments (the boy scouts now brought in a rough wooden model of a coelostat and telescope-camera and set them up on boxes representing piers). You see the astronomer proposes to use a coelostat—this mirror is to reflect the Sun's light into the camera. There is the Sun on our lantern-screen, and you see that the eclipse has already begun, for the edge of the Moon has taken a small bite out of the Sun's disc. It takes about an hour to cover the whole disc; we will not keep you so long as that, but while it is moving slowly across I will tell you what these gentlemen propose to do. The camera is pointed to the coelostat mirror in exactly the right position; and the astronomer is to take five photographs with different exposures. The first exposure is to be only for one second; this will suffice to show the brightest parts of the corona though not the faint portions; but even if the
