254 Mr. W. H. Dines. [June 19, 



The older books on engineering give P = 0'005t> 3 , but more recent 

 books give P = OOOSv 2 . 



It is a lower value than the one determined at Hersham last year, 

 but I had then no suspicion that the frame of the apparatus would 

 influence the result. It is, however, borne out by the values 

 obtained last year for smaller plates, and the experimental evidence 

 which shows that a decrease of pressure per sq. ft. occurs as the size 

 of the plate is increased. 



The following particulars may also be of interest ; the method by 

 which they were obtained is described in a paper read before the 

 Royal Meteorological Society in May, 1890. At the centre of the 

 plate when exposed normally the increase of pressure at a rate of 

 60 miles an hour is equal to 1'82 inches of water, and the decrease at 

 the back, also at the centre close to the plate, is equal to 0*89 inch 

 of water. These values were found with the plate at the point B 

 (see preceding diagram), and, taken in connexion with the fact, 

 discovered I believe by Mr. Curtis, that the pressure in front 

 decreases from the centre outwards, agree fairly well with the value 

 for the pressure obtained in that position. 



Long Narrow Vane. 



Experiments have also been made with long narrow strips instead 

 of with a square plate. The size chosen was 4 feet long by 3 inches 

 broad, the surface thus being the same as the square plate. 



Observations were made at angles of 10 apart, in positions I and 

 II, both with the shorter axis inclined to the wind, and also with the 

 longer axis. 



There was a difficulty in mounting these strips so that the 

 supporting arm should not cause any disturbance of the motion of 

 the air over the strip, and still be sufficiently rigid to support the 

 pressure. The thickness of the wood was inch, and the edges were 

 feathered off. It will be seen that when the strip was exposed so 

 that its shorter axis was inclined to the wind the longer axis was 

 necessarily parallel to the long arm of the whirling machine, and its 

 centre 2 feet from the end of the lever. Under these circumstances 

 the velocity would be nearly 4 per cent, greater, and the pressure 

 from 7 8 per cent, greater, than upon the square plate for the same 

 rate of rotation of the whirling machine. This has been taken into 

 account in drawing the curve Gr. 



The support was obtained by a piece of flat iron, l inch by ^, 

 which passed half way along the back of the wood, the end of the 

 iron being bolted to the lever. 



The values of the moment for position I were found to be greater 

 than for position II, doubtless on account of the eddy from the 



