8o 



NATURE 



[Mav 28, 1908 



assumed structure of the wind the small plate might 

 register a pressure lower than the large one. This 

 is the somewhat rare event when in a gale there is 

 one gust of considerably greater intensity than those 

 which precede or follow it. If during this gust 

 the small plate occupied a region of low velocity, its 

 registered maximum pressure would be lower than that 

 of the large plate. This appears to have happened 

 in one of the gales at the Forth Bridge during the 

 experiments, but its rarity supports the evidence of 

 anemometers, which show that the average gale con- 

 sists of a series of gusts of nearly equal intensity, so 

 that the probability of the maximum velocity occurring 

 in the region of the small plate is very great. It is 

 important to realise that the above conclusions are in 

 no respect applicable to the pressures which may ob- 

 tain at any given instant on two plates during ;i 

 gale, as a little consideration will show that the prob- 

 ability of the small plate occupying the region of 

 lowest velocity at any instant is the same as that of 

 its occupying the region of highest velocity, from 

 which the conclusion follows that the small plate will 

 also register the lowest pressure during the gale. 



In the foregoing statements the difference in re- 

 sistance of large and small surfaces in the wind has 

 been treated as depending entirely on the structure of 

 the wind, that is, it has been assumed that if the 

 wind were a perfectly uniform current in which the 

 velocity over any considerable area was the same, 

 the pressures on the two surfaces woidd be identical. 



This, of course, is not necessarily the case, as there 

 may exist a purely dimensional eff-ct in the resistance 

 of appreciable magnitude, and in the opinion of some 

 authorities the explan.'ition of the Forth Bridge ex- 

 periments was to be found in this, and not in the 

 structure of the wind. 



For this reason, when the wind-pressure experiments 

 were commenced at the National Phvsical Laboratorv 

 in 1904, the determination of the existence or non- 

 existence of this dimensional effect was made the chief 

 feature of the research. These experiments were 

 made on ])lates and models ranging up to 100 square 

 teet in area, erected on the top of an observation 

 tower 50 feet above the ground, which had a fairly 

 clear space in front of it. .After some preliminary- 

 work, the method which was finally adopted consisted 

 in the determin.ition of the constant fe in what may 

 be called the " equivalent " pressure velocity relation 



that is the relation which would exist if the velocity 

 of the wind were uniform. 



The determination of this relation when a plate is 

 moved at a known velocity in still air is fairly easy. 

 It becomes more dilTicull when a plate is suspended 

 in a uniform current of air on account of the trouble 

 involved in forming a correct estimate of the velociiv 

 of the current, since, owing to the conditions of flow 

 being disturbed in the region of the plate, it is neces- 

 sary to place the velocity gauges at some distance 

 from the plate. In the case of a plate exposed to the 

 wind, there is the added complicalion of the varying 

 structure of the current, and the problem at the 

 National Physical Laboratory was to obtain the 

 " equivalent " pressure velocity relation from observ- 

 ations of the resultant pressure on a plate and the 

 corresponding pressure in a " Dines " tube, which 

 was used as the velocity gauge, distant 10 feet from 

 the edge of the plate. A solution was found in the 

 observed fact that although the pressures at any in- 

 stant in two tubes facing the wind, and distant 10 

 feet apart, might differ hy as jnuch as 50 per cent., 

 yet if one hundred of these sets of readings were taken 

 at successive intervals of time, the mean pressures for 

 NO. 2013, VOL. 78] 



each tube were practically identical. From this it was 

 assumed that if a large number of observations of 

 the resultant pressure on the plate and the (simul- 

 taneously observed) pressure in the " Dines " tube 

 were made, the means of these experiments would give 

 the equivalent pressure-velocity relation sought. For 

 this purpose 200 observations of this kind were made 

 on each plate tested, two observers operating two 

 sensitive water gauges at the fcot of the tower. One 

 of these water gauges was connected by two lead 

 pipes attached to the legs of the tower to the " Dines " 

 tubes, and the other by two similar pipes to an air 

 cylinder in which the pressure varied with the fluctu- 

 ations of resultant pressure on the plate. The arrange- 

 ment of the loo-square-feet plate and the " Dines " 

 tube is shown in the photograph (Fig. i). 



The results of experiments on three plates of areas 

 of 25, 50 and 100 square feet gave identical values of 

 the constant /: in the " equivalent " pressure-velocity 



Fl(.. I. — wind Obser\ation Tower with loo-squarc-feel plate in position. 



relati(.)n, which in units of |)ounds per square foot and 

 miles per hour was found to be o'oo32, indicating that 

 for this range in dimensions the purelv dimension.il 

 effect in the resistance was negligible. There wen' 

 strong reasons, however, to suppose that it was not 

 negligible for all ranges in dimensions, since the value 

 of k determined at the National Physical Laboratorv 

 in iqoj for jjlates of 2 and 3 square inches in 

 area in a uniform current was 00027, and that deter- 

 mined by Mr. Dines in i8go for a plate of i square 

 foot in a whirling machine was 00029. This view 

 has been fully confirmed by the publication during thr 

 present year of the results of M. Eiffel's experimencs 

 on plates let fall from the second stage of the Eiffel 

 Tower. Using square plates varying in area front 

 10 square feet to five-eighths of a square foot, M. 

 Eiffel found a continuous change in the value of tlT- 



