Pret^sure around Spheres in a Viscous Fluid. 429 



5. Distribution of Pressure around, a Single Sphere. — A 

 glass sphere five millimetres in diameter with an opening 

 two-tenths millimetre in diameter was mounted in the tube ; 

 the opening was in the equator whose plane was parallel to 

 the direction of the stream-lines in the tube, and could be 

 rotated through 360° in this plane (fig. 2). The pressure over 

 unit surface of the sphere at all points in this equator could 

 then be observed : and since by symmetry this plane is 

 identical with any other equatorial plane parallel to the same 

 straight lines, the total distribution of pressure around the 

 sphere may be obtained by rotating the pressure-distribution 

 curve obtained in this plane through 180° around an axis 

 parallel to the axis of the tube. 



The full-line curve mm, Plate XVIII. fig. 9, shows the 

 distribution of pressure in a plane parallel to the stream- 

 lines, in terms of the pressure normal to the surface of the 

 tube. 



In all the diagrams, unless otherwise stated, the curves of 

 the observed pressures are plotted to a scale in which the 

 pressure of one millimetre of water is represented by each 

 o mm. circle measured from the double circle marked a.p. in 

 the diagram. 



6. Pressure around two Spheres lohose Line of Centres is 

 Parallel to the Stream-lines. — Two similar spheres of 5 mm. 

 diameter were placed in the tube with the line of their 

 centres parallel to the direction of flow, the distance apart 

 of their surfaces being 1*5 cm. They were first placed with 

 their openings up stream, making ^ = (fig. 5). The openings 

 were then rotated through an angle of 180°. Readings of 

 the pressure normal to the surface of the spheres, as given 

 by the water-manometer m, were taken for each 15°, The 

 velocity of the air-current, as measured hj the pressure at 

 the ends of the tube, being the same for each reading. 



In the following table columns 2 and 3 give the pressure 

 normal to the surface of the sphere, in millimetres of water, 

 for a normal pressure in the tube of three millimetres of water. 

 Columns 4 and 5 give the same for a pressure in the tube of 

 one and eight-tenths millimetres of water. 



The pressure diagrams plotted from these readings are 

 exhibited in fig. 10, 



The general form for the pressure-distribution around 

 Sphere A is similar to that for a single sphere. The distri- 

 bution around B is slightly modified by the presence of A. 

 The distributions for the two normal pressures (curves mm 

 and m^m') are similar. 



