No. LX.—THE FIFTEEN BRIDGES
It is possible to pass over all the bridges which connect the islands A and B and the banks of the surrounding river without going over any of them twice.
The course can be shown thus, using capital letters for the different regions of land, and italics for the bridges:—Ea Fb Bc Fd Ae Ff Cg Ah Ci Dk Am En Ap Bq ElD.
This order of the bridges can, of course, be reversed.
No. LXVI.—A DUCK HUNT
In order that a spaniel starting from the middle of a circular pond, and going at the same pace as a duck that is swimming round its edge, shall be sure to catch it speedily, the dog must always keep in the straight line between the duck and the centre of the pond.
The duck can never gain an advantage by turning back, and if it swims on continuously in a circle it will be overtaken when it has passed through a quarter of the circumference, for the dog will in the same time have described a semi-circle whose diameter is the radius of the pond, ending at the point where the duck is caught.