10 



HYDRAULICS AND ITS APPLICATIONS 



If </ be measured in inches, this artificial elevation in inches is given by 



04624 . 



/* = 



inches. 



In the case of a mercury column, taking the specific gravity of mercury 

 t, be 1 :v.v,);, we have cos a = -0276 



4 X -03698 X '6276 X 144 . 



62-4 X 13-596^- ' mchefl 



= _ inche8 



(I 

 where d is measured in inches. 



The negative sign here indicates that the surface of the mercury 

 column is depressed below the level corresponding to the statical pressure 

 alone. The following table gives values of h at 68 F. corresponding to 

 various tube diameters. 



DIAMETER OF TUBE IN INCHES. 



Evidently then, tubes of small bore below about '3 in. internal 

 diameter are not advisable for use in such pressure-measuring apparatus, 

 especially where small pressures are to be measured, except where the 

 difference of height in two similar tubes is used to indicate the pressure. 

 Here the effects of capillarity, being the same for each tube, neutralise 

 each other. Again, since the specific gravity of mercury is 13*596, the 

 proportional error introduced by neglecting capillarity in the case of a 



mercury column will be 



13-596 X '01576 



= 4-63 times that introduced 



04624 

 with a water column, and will be in the opposite direction. 1 



The effect of surface tension in liquid in motion may be strikingly 

 shown by allowing two jets of water, moving steadily in the same straight 



k of space prevents any further investigation into the properties of surface films. 



These are, however, except in so far as they affect the stability of jets, only of slight impor- 



tani-i- iii hydraulics. For further information the reader is referred to any textbook of 



<>i hydrostatics, or to the article on Capillary Action in the " Encyclopaedia 



Britannica." 



