Page:Popular Science Monthly Volume 14.djvu/852

832 time to collect and note the facts. Here is a table showing the readings of the Nilometer for thirteen years, from which it is clear that the river Nile, during that time at least, does not confirm the rule, "Maximum spots, maximum rainfall":

The Vertebral Articulations in Birds.—Professor Marsh, in the "American Journal of Science and Arts" for April, essays an explanation of the peculiar saddle-shaped articulation seen in the vertebræ of birds. Between Ichthyornis and Hesperornis, two birds with teeth from the Cretaceous, there is the widest conceivable difference as regards this part of the skeleton, in Hesperornis the ends of the centrum being saddle-shaped, as in ordinary birds, while in Ichthyornis the articulation of the centrum is cup-shaped. But in the third cervical vertebra of Ichthyornis, Professor Marsh catches nature in the act, as it were, of forming a new type, by modifying one form of vertebra into another. Following this hint, the connection between these widely divergent types of structure soon becomes apparent, and the development of the modern form of avian vertebra from the fish-like biconcave form finds a solution. In the anterior articulation of this vertebra of Ichthyornis the surface looks downward and forward, being inclined at an angle of nearly 60° with the axis of the centrum. In vertical section it is moderately convex, while transversely it is strongly concave, thus presenting a near approach to the saddle-like articulation. None of the other vertebræ of Ichthyornis possesses this character. "This highly specialized feature," remarks Professor Marsh, "occurs at the first bend of the neck, and greatly facilitates motion in a vertical plane. If, now, we consider for a moment that the dominant motion in the neck of a modern bird is in a vertical plane, we see at once that anything that tends to facilitate this motion would be an advantage, and that the motion itself would tend directly to produce this modification. With biconcave vertebræ, the flexure in any direction is dependent on the elasticity of the fibrous tissue that connects them, as the edges of the cup do not slide over each other. An increasing movement in the neck of Ichthyornis in a vertical plane would tend to deflect the upper and lower margins of the circular cup, and to produce a vertical constriction, and at the same time to leave the lateral margins projecting; and this is precisely what we have in the third vertebra. This modification of the vertebræ would naturally appear first where the neck had most motion, viz., in the anterior cervicals, and gradually would be extended down the neck; and, on to the sacrum, if the same flexure were continued. Behind the axis, or where the vertical motion prevails, we find in modern birds no exception to the saddle articulation in the whole cervical series. In the dorsal vertebræ, this cause would be less efficient, since the ribs and neural spines tend to restrict vertical motion, and hence to arrest this modification. This region, then, as might be expected, offers strong confirmatory evidence of the correctness of the above explanation; for here occur, among modern birds, the only true exceptions known in the presacral series to the characteristic saddle-shaped articulation."

Professor Tyndall on Sound.—Professor Tyndall is this season giving a course of lectures on sound at the London Royal Institution. In the first lecture he illustrated by many experiments the action of sound-waves, and explained the mechanism of the ear. In treating of the velocity of sound, he said that at the temperature of 32° Fahr. air conveys sound-waves 1,090 feet per second, but that this rate varies with every variation of the temperature. It is well known that, when a mechanically striking bell is placed under a receiver exhausted of air, no sound is heard. Professor Tyndall showed by experiments that when a little air, about one fourth, is admitted into the receiver, the sound is feeble only; but on introducing a little hydrogen, the sound was again stilled. This fact was known to