xxxviii GENERAL SUMMARY OF SCIENTIFIC AND 



and steel may be increased by alternately straining them up 

 to the limit of elasticity, and then removing the pressure. 

 Similar observations have been made by Commander L. A. 

 Beardslee, United States Navy. Neissen has shown that the 

 elastic reaction of torsion increases with the duration of the 

 experiment. 



At the meeting of the National Academy of Sciences, in Phil- 

 adelphia, Professor Rood described the modifications he had 

 made in the arrangement of Zollner's horizontal pendulum, 

 which have rendered the instrument fit for use in such phys- 

 ical investigations as require the measurement of very minute 

 changes in dimension. He has thus succeeded in measuring, 

 with the modified apparatus, so small a quantity as twenty 

 billionths of an inch. 



Marey continues his beautiful researches in Animal Me- 

 chanics ; and during this year has solved what has heretofore 

 been the most abstruse point in the mechanical theory of 

 flight. He has, in fact, experimentally shown that the re- 

 sistance opposed to the bird's wings during flight is far 

 greater than the resistance opposed when the bird first rises 

 on the wing to began its flight: for in the former case the 

 bird's wing always beats a new portion of air which it tends 

 to depress; but, on account of the short duration of the 

 pressure which it receives, any one of these portions of air 

 has not the time to acquire the velocity of the wing ; these 

 portions of air are therefore successively compressed, and 

 offer the maximum or initial resistance to the wing. Marey 

 has also made an interesting research on human locomotion, 

 in which he shows that the leg in walking does not, as Weber 

 maintains, swing with pendulous motions, but is animated 

 with nearly uniform motion while the foot is off the ground. 



In Acoustics, Professor Tyndall has made observations off 

 the coast of England on the transmission of the sounds of 

 fog-horns and cannon through the atmosphere when in dif- 

 ferent conditions. He is of the opinion that he has estab- 

 lished the very interesting and important fact that in clear 

 weather, when the sun causes a rapid evaporation of the 

 water of the ocean, the air is rendered less permeable to 

 sound, by reason of the reflection of the sonorous waves from 

 the surfaces of portions of air differing considerably in their 

 temperatures and in their degrees of hydration. On the 



