35 Trans. Zool. Soc. IV, 1850, p. 27.
36 It would seem to be a common if not a general rule that marine organisms, zoophytes, molluscs, etc., tend to be larger than the corôÙreôÙsponôÙding and closely related forms living in fresh water. While the phenomenon may have various causes, it has been attributed (among others) to the simple fact that the forces of growth are less antagonised by gravity in the denser medium (cf. Houssay, La Forme et la Vie, 1900, p. 815). The effect of gravity on outward form is illustrated, for instance, by the contrast between the uniformly upward branching of a sea-weed and the drooping curves of a shrub or tree.
37 The analogy is not a very strict one. We are not taking account, for instance, of a proportionate increase in thickness of the boiler-plates.
38 Let L be the length, S the (wetted) surface, T the tonnage, D the displacement (or volume) of a ship; and let it cross the Atlantic at a speed V. Then, in comparing two ships, similarly constructed but of different magnitudes, we know that L =ã₤Vÿ£¢2ã₤, S =ã₤Lÿ£¢2 =ã₤Vÿ£¢4ã₤, D =ã₤T =ã₤Lÿ£¢3 =ã₤Vÿ£¢6ã₤; also R (resistance) =ã₤Sã₤ôñã₤Vÿ£¢2 =ã₤Vÿ£¢6ã₤; H (horse-power) =ã₤Rã₤ôñã₤V =ã₤Vÿ£¢7ã₤; and the coal (C) necessary for the voyage =ã₤Hã₤ãã₤V =ã₤Vÿ£¢6ã₤. That is to say, in ordinary engineering language, to increase the speed across the Atlantic by 1 per cent. the shipãs length must be increased 2 per cent., her tonnage or displacement 6 per cent., her coal-consumpt also 6 per cent., her horse-power, and therefore her boiler-capacity, 7 per cent. Her bunkers, accordingly, keep pace with the enlargement of the ship, but her boilers tend to increase out of proportion to the space available.