Chap. XXXI .
On Long and Round Stones.
P ieces of iron join more firmly with a long stone than with a round one, provided that the pole of the stone is at the extremity and end of its length; because, forsooth, in the case of a long stone, a magnetick is directed at the end straight towards the body in which the virtue proceeds in straighter lines and through the longer diameter. But a somewhat long stone has but little power on the side, much less indeed than a round one. It is demonstrable[180], indeed, that at A and B the coition is * stronger in a round stone than at C and D, at like distances from the pole.