S. jP. fangley — Internal Work of the Wind. 41 



the details of this section in southeastern Pennsylvania is an 

 interesting problem, left for. solution to some geologist who has 

 the necessary paleontologic training and who will not be dis- 

 couraged by the prospect of a good deal of hard work before 

 the desired result can be obtained. 



The problem of where to draw the line in this series of lime- 

 stones, on a geological map, between the Cambrian and Ordo- 

 vician, is one that will seriously embarrass the geologist, but I 

 anticipate that either lithologic or paleontologic characters will 

 be discovered by which the two groups can be differentiated. 

 If not, the limestones must be colored as one lithologic unit or 

 formation and the approximate line of demarkation between 

 the Cambrian and Ordovician indicated in the columnar sec- 

 tion accompanying the legend of the map. 



Art. V. — The Internal Work of the Wind;* by 

 S. P. Langley. (With Plates I-Y). 



Part I. — Introductory. 



It has long been observed that certain species of birds main- 

 tain themselves indefinitely in the air by " soaring," without 

 any flapping of the wing, or any motion other than a slight 

 rocking of the body; and this, although the body in question 

 is many hundred times denser than the air in which it seems 

 to float with an undulating movement, as on the waves of an 

 invisible stream. . 



No satisfactory mechanical explanation of this anomaly has 

 been given, and none would be offered in this connection by 

 the writer, were he not satisfied that .it involves much more 

 than an ornithological problem, and that it points to novel 

 conclusions of mechanical and utilitarian importance. They 

 are paradoxical at first sight, since they imply that under cer- 

 tain specified conditions, very heavy bodies entirely detached 

 from the earth, immersed in, and free to move in, the air, can 

 be sustained there indefinitely, without any expenditure of 

 energy from within. 



. These bodies may be entirely of mechanical construction, as 

 will be seen later, but for the present we will continue to con- 

 sider the character of the invisible support of the soaring 

 bird, and to studv its motions, though only as a pregnant 



* A paper read (by title only) to the National Academy of Sciences, in April, 

 1893. and subsequently (in full) at the Aeronautical Congress, at Chicago, in 

 August, 1893. 



