832 



THE POPULAR SCIENCE MONTHLY. 



time to collect and note the facts. Here is 

 a table showing the readings of the Nilome- 

 ter 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 " : 



YearB, Depth of the Nile. 



1866 2Si feet minimum spots. 



1867 24i " miuimum spots. 



1868 19 " 



1669 29i " 



18TO 2£i " 



1871 -23* " 



1872 25i " maximum spots. 



1873 20 " 



1874 29 " 



1875 24 " 



1876 ...25 " 



1877 18 " minimum spots. 



1878 30 " 



The Vertebral Artienlations ia 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 vertebra of birds. 

 Between Ichihyornis and Hesperornis, two 

 birds with teeth from the Cretaceous, there 

 is the widest conceivable difference as re- 

 gards this part of the skeleton, in Hesper- 

 ornis the ends of the centrum being saddle- 

 shaped, as in ordinary birds, while in Ich- 

 thyornis the articulation of the centrum is 

 cup-shaped. But in the third cervical verte- 

 bra of Ichthyornis, Professor Marsh catches 

 nature in the act, as it were, of forming a 

 new type, by modifying one form of verte- 

 bra 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 ar- 

 ticulation of this vertebra of Ichthyornh 

 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 trans- 

 versely it is strongly concave, thus present- 

 ing a near approach to the saddle-like ar- 

 ticulation. None of the other vertebrae of 

 Ichthyornis possesses this character. " This 

 highly specialized feature," remarks Pro- 

 fessor 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 



facihtate this motion would be an advan- 

 tage, and that the motion itself would tend 

 directly to produce this modification. With 

 biconcave vertebrae, the flexure in any di- 

 rection 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 project- 

 ing ; and this is precisely what we have in 

 the third vertebra. This modification of 

 the vertebrae would natuially 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 con- 

 tinued. Behind the axis, or where the ver- 

 tical motion prevails, we find in modern 

 birds no exception to the saddle articula- 

 tion in the whole cervical series. In the 

 dorsal vertebrae, this cause would be less 

 efiicient, 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 excep- 

 tions 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 In- 

 stitution. 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 sec- 

 ond, 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 Tyn- 

 dall 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 10 



