WIND PRESSURE. 577 



surfaces which in size and shape corresponded to the projections of 

 the solids in question. 



For ihis purpose it was necessary first to obtain an artificial 

 wdnd, a current of air of uniform or nearly uniform velocity and 

 direction. For this purpose an accurately formed screw propeller 

 with four blades of 28in. diameter and 40in. pitch was prepared 

 and placed at one end of a tube 30in. diameter and 36in. long, and 

 driven at a high speed by means of the Otto gas engine, at the 

 engineering laboratory of the University. It was hoped that this 

 would give a uniform blast, but such was not found to be the case. 

 Instead thereof it produced a cylindrical shell about 6in. thick of 

 helically moving air surrounding a central core of dead or motion- 

 less air. After a number of experiments it Avas found that by 

 placing a longitudinal radial diaphragm in the tube, and by adding 

 at the end a peculiar funnel-shaped attachment, the axis of which 

 was a tangent to the helical path of one particular portion of the 

 air, it was possible to obtain a jet of air of fairly uniform direction 

 and velocity, having a cross section of 12in. by lOin., or an area of 

 120 sq. in. This jet of air was tested in various ways, the direction 

 being studied by the aid of a small flag, while the velocity was 

 given by means of a Revy's current meter, and checked by a 

 counter attached to the axis of the propeller. 



To determine the pressure on surfaces or solids of various forms 

 the following arrangement was used : — A large and new engineer's 

 surface plate was placed on a very firm support, cut off as far as 

 possible from the vibration of the machinery, and accurately 

 levelled. On this was placed a very carefully made three-wheeled 

 carriage with gunmetal wheels turning on very fine steel centres. 

 This carriage had been originally made for delicate physical experi- 

 ments, and belonged to the physical laboratory of the University. 

 The surface plate and carriage were cased in, so as to be protected 

 from the wind, and from the carriage a thin stem rose up, passing 

 through a slot in the casing and terminating in the centre of the 

 air jet. On this stem were carried the surfaces and solids experi- 

 mented with. The former of these w^ere made of stout cardboard, 

 while the latter were in most cases geometrical models belonging 

 to the physical department of the University. The force of the 

 wind was measured by a delicate spring balance, which, prior to 

 use, was tested by means of a set of standard weights. The 

 ajDparatus worked well and gave no trouble. Its indications were 

 fairly precise, the wind fluctuating but very little owing to the gas 

 engine being provided with a large flywheel, and there being also 

 a second flywheel on a countershaft between the engine and the 

 blowing apparatus. 



The first experiments were made to determine the law connect- 

 ing the velocity and pressure. A variety of tests were made with 

 discs, spheres, cubes, and rectangular plates, the gas engine being 

 made to run alternately as fast and as slow as possible by altering 

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