494 Mr. H. Wilde on the Efflux of Air as 



When Mr. Aitken places his shallow trays of thin metal 

 "over the ground to be tested/' are we to understand that 

 the edges of the trays are in contact with the ground ? If so, 

 the results are not in harmony with another set of obser- 

 vations, in which two slates or two iron weights are placed 

 one on the ground and the other elevated a few inches above 

 it. That on the ground, " and in heat communication with 

 it," is said to have remained dry, while the elevated one 

 was bedewed all over. Surely this result is in favour of the 

 received theory. 



Perhaps the most startling portion of the new theory is 

 that which relates to plants, whose very existence would 

 seem in many cases to depend on the formation of dew. If 

 the moisture on plants were " the excretion of liquid " by 

 the plants themselves, and not dew, for what purpose are 

 plants endowed with such high radiating powers, in some 

 cases superior to that of lampblack? Taking this at 100, 

 Melloni, by his careful method, found the radiating power of 

 different herbs with flexible leaves to be 103, the leaves of 

 the elm and poplar to be 101, that of vegetable mould 92. 



Enough, however, has been said on this subject; but we 

 cannot conclude without offering an apology to the readers 

 of the ' Philosophical Magazine ' for the very elementary 

 details of this article. 



LXVIII. — On the Efflux of Air as modified by the Form of 

 the Discharging Orifice. By Henry Wilde, Esq* 



IN my former paper on the Efflux of Air, the hydraulic 

 coefficient '62, as commonly applied to the discharge of 

 ' elastic fluids through an orifice in a thin plate, was taken 

 as the value of the contraction of such orifice, and from this 

 coefficient the highest velocities shown in the several tables 

 were deduced. A review of the results of my experiments 

 by Prof. Osborne Reynolds f led me to doubt the value of this 

 coefficient, and to make further experiments with the object 

 of determining the maximum rate of discharge from an 

 orifice of the best form. 



Five disks of brass had each a hole drilled through its 

 centre two hundredths of an inch in diameter. Equality in 

 the size of the holes was accurately determined by means of 

 a standard cylindrical gauge. These disks I shall designate 

 A, B, C, D,E. 



* Communicated by the Author, having been read at a Meeting of the 

 Manchester Literary and Philosophical Society, March 23rd, 1886. 



t Proceedings Manchester Lit. and Phil. Society, vol. xxv. p. 55 j Phil. 

 Mag. March 1886, p. 185. 



