Acacia 15,467 4,840 4,436 Fir 15,218 4,382 2,572 Beech 10,965 6,028 4,643 Oak 12,622 5,036 4,229 Pine 10,900 4,611 2,605 Elm 13,516 4,665 3,324 Sycamore 14,639 4,916 3,728 Ash 15,314 4,567 4,142 Alder 15,306 4,491 3,423 Aspen 16,677 5,297 2,987 Maple 13,472 5,047 3,401 Poplar 14,050 4,600 3,444 Separating a cube from the bark-wood of a good-sized tree, where the rings for a short distance may be regarded as straight: then, if A R, Fig. 14, be the section of the tree, the velocity of the sound in the direction _m n_, through such a cube, is greater than in the direction _a b_. [Illustration: FIG. 14.] The foregoing table strikingly illustrates the influence of molecular structure. The great majority of crystals show differences of the same kind. Such bodies, for the most part, have their molecules arranged in different degrees of proximity in different directions, and where this occurs there are sure to be differences in the transmission and manifestation of heat, light, electricity, magnetism, and sound. § 14. _Hooke’s Anticipation of the Stethoscope_ I will conclude this lecture on the transmission of sound through gases, liquids, and solids, by a quaint and beautiful extract from the writings of that admirable thinker, Dr. Robert Hooke. It will be noticed that the philosophy of the stethoscope is enunciated in the following passage, and another could hardly be found which illustrates so well that action of the scientific imagination which, in all great investigators, is the precursor and associate of experiment: