ON STREAM-LINE MOTION OF A VISCOUS FLUID. 137 



conditions of velocity of flow in questions of such importance as dis- 

 continuity of fluid motion and viscosity. The second purpose for which 

 this might be employed was obviously to investigate the nature of stream 

 line forms in many cases in which mathematical investigation was 

 impossible, not merely for the case of flowing water, but in applications to 

 heat and electricity. 



It was in experimenting with various liquids with the first mentioned 

 object in view that much better results were obtained than those given 

 with water, and after working out the test case with various fluids 

 (including water) under new and more rigorous conditions, these results, 

 together with certain new experiments, are brought before the Associa- 

 tion. 



(2.) Description of Improved Apparatus. 



In the earlier apparatus the main body of water was supplied in a 

 thin sheet by the pressure from the mains, coloured water being intro- 

 duced from a small reservoir kept under pressure by means of a hand 

 pump. Since exhibiting these experiments in the earlier part of the year 

 at the Royal Society, improvements have been made both in the general 

 mode of applying the fluid to the lantern apparatus, and also in the 

 lantern apparatus itself, thus rendering the appliance suitable for either 

 physical or engineering lecture purposes, as well as for actual experi- 

 mental work. 



The arrangement consists of a lantern and two vessels, one containing 

 clear liquid and the other coloured liquid, connected by two pipes with 

 the lantern-slide. A pipe leads to the reservoir of compressed air, which 

 is attached to a circular cap, with which the vessels of liquid are con- 

 nected. Taps enable the connection between the lantern- slide and 

 receiving- vessels to be adjusted, the annular pipe with which the air is 

 connected passing down to the bottom of the connecting vessels, whereas 

 the taps are so arranged that the pressure of air from the reservoir comes 

 upon the surface of the liquid in each vessel. The containing vessels - 

 which have been used up to the present are ordinary glass aerated water- 

 bottles, capable of sustaining 200 lb. per square inch. At the head of 

 the pipe on each containing- vessel a separate pressure gauge can be 

 attached, as well as on the reservoir itself, so that the pressures can he 

 adjusted accurately for any particular experiment. If different liquids 

 are required to be used, they can be connected with the circular head, 

 without the necessity of disconnecting the other containing- vessels. 



The chief object with which this arrangement was designed, however, 

 was to enable high pressures, such as from 1001b. to 200 lb. per square 

 inch, to be employed when very thin sheets of liquid are used, a high 

 pressure being necessary under such conditions in order to insure the 

 flow. 



With regard to the lantern slide itself, the original apparatus, although 

 efiective, was troublesome to make and manipulate, and did not insure 

 absolute uniformity of the thin sheet, or lend itself to rapid changes. 

 The new device merely consists of a small brass block containing two 

 chambers. It has two pipes projecting from it, communicating with the 

 chambers, one pipe being connected with the vessel of clear, and the other 

 with the vessel of coloured, fluid. The small brass block is merely 

 inserted between two plates of glass, together with a third exactly the 

 same thickness as itself. By then making in thin cardboard, paper, lead 



