both the ways we described; and besides that, the distances of some of them (especially Sirius) have been measured; and all the facts fit in well together, so that we can actually make a model of this system of stars. Miss Bellamy has kindly made one for us by sticking hatpins into this cork mat: and please notice how very flat it is. The pinheads are nearly all at the same height from the mat—not quite the same, but nearly enough to be note-
worthy. We may almost compare this group of stars to a fleet of ships sailing on the flat ocean: but the fleet is of a stupendous size—its length is about 150 light years; that is to say that if the admiral on the leading ship signalled to his fleet, it would be 150 years before the last ship would even see the signal! How long it would take us to go round and pay a visit to each ship in turn you can perhaps imagine for yourselves.
We have now noticed three groups of stars and found that, though they may be altering their places a little in the group, each group is never-