A VOYAGE IN SPACE
further and further north, and will disappear after 1932. There are altogether at any one time twelve such "families" of eclipse tracks; six of them are travelling northwards and six are travelling southwards. Every now and then one of them goes out at one pole or the other; but there is always a new family born about the same time to keep the number of twelve families complete. A few years ago, in 1909, a new family was born at the North pole which is of special interest to us. At its next return, in 1927, the track will come much further south and will cross the north of England (Fig. 77). So in thirteen years time you will have a chance of seeing a total eclipse without leaving England; you are very fortunate young people, for your parents and grandparents and greatgrandparents have had no such chance since 1724, nearly 200 years. You must be careful to be ready for it; I think you will only have twenty-five seconds of total eclipse; but still that will be long enough to give you a good view of the beautiful corona.
When the time is so short (twenty-five seconds is specially short, but even the longest total eclipse only lasts a few minutes) it is naturally important to make the most of it. Photography has helped us considerably in doing so, because we can arrange our cameras beforehand in exactly the right positions, and we can drill those who are to take the photographs until they can make the exposures quickly and without a mistake. A delay or a mistake would waste the precious seconds terribly; but the force of habit is so strong that after going