NA TURE 



25 



THURSDAY, NOVEMBER 14, 1878 



:leopatra s needle and 



PRESSURE 



THE WIND 



I 'T'HE statements recently made in the Times respecting 

 the stability of Cleopatra's needle and the maximum 

 tensity of the pressure of the -wind in this country have 

 awakened much interest, if not anxiety, about the sub- 

 iect. The appearance of the lofty obelisk balanced on so 

 lall a base suggests to many the thought of an egg 

 .nding on its end, and presents every idea of insta- 

 !;ty. This idea is much amplified by a very erroneous 

 rimation, we believe, by most persons of the real 

 inensions of the base; we hare heard this estimated at 

 arious diameters do^vn to two feet, but in reality it is 

 in no direction less than five. The statement that the 

 stability of the obelisk is sufficient to withstand a wind 

 pressure of 80 or 90 lbs. per square foot having been 

 made, the storm from Liverpool at once broke on it and 

 upset people's minds, if not the monolith. Thus we 

 learn, from the observations taken by Mr. Hartnup, the 

 astronomer at the Liverpool Observatory, that on January 

 10, 1868, "it began to blow strongly about 9 A.M., and 

 from that time gradually increased in violence until half- 

 past II P.M. on the 31st, when there was one gust of 

 'nd which registered 51 lbs. on the square foot. From 

 5 time the gale rapidly increased till noon next day, 

 wing with a severity quite unprecedented in this 

 untry. The anemometer which has been erected at 

 the Bidston Observatory is made to register up to 60 lbs. 

 on the square foot, the idea being that no gale would 

 reach that degree of violence. Between eleven o'clock 

 "d one o'clock, however, the registering pencil was 

 :ven far beyond this limit, and Mr. Hartnup calculated 

 t.at at several periods the pressure could not have been 

 less than from 70 lbs. to 80 lbs. on the square foot. The 

 emometer was erected in 1851, and the most severe 

 le registered up to this time was in December, 1863, 

 when there were three gusts which registered 45 lbs. to 

 the square foot." Further details respecting this re- 

 markable hurricane will be found in the Journal of the 

 Scottish Meteorological Society, from which we find 

 that at Glasgow, from 1.15 P.M. to 1.30 P.M., twenty-one 

 miles of wind passed the observatory, giving a velocity 

 of eighty- four miles per hour, or corresponding to a pressure 

 ' 35j lbs. to the square foot, while the strongest gusts 

 istered 42 lbs. on the square foot. At Edinburgh the 

 e was more severe than at the latter place ; cabs and 

 rses are said to have been blown over, but there is no 

 ord of the pressure or velocity as there was unfor- 

 lately no anemometer in working order. Many autho- 

 es state that the maximum pressure of the wind does 

 ...A exceed 55 lbs. to the square foot in this country, and 

 as this is the figure commonly assumed by engineers in 

 the design of large structures, it is of the greatest import- 

 ance that the trustworthiness of the Bidston anemometers 

 ould be ascertained. Pressure anemometers are ob- 

 usly liable to errors from the varying modulus of 

 elasticity of their springs and the momentum of their 

 moving parts and supports, while Robinson's anemometers 

 may give a maximum velocity due to small eddies, which 

 VoT, riy,— No. .72 



is much in excess of the true value. Mr. John Dixon in 

 his letter to the Times on the subject gives a good illus- 

 tration of a pressure of 80 lbs. to the square foot by com- 

 paring it to the weight on the floor of a densely-crowded 

 room. It has been ascertained by experiment that the 

 weight of a crowd of persons can attain 80 to 120 lbs. per 

 square foot, the latter figure being reached only when the 

 experiment was made with labourers of above the average 

 stature packed as closely as possible, and the former being 

 commonly taken as the maximum load to which the plat- 

 form of a bridge can be subjected by a dense mixed 

 crowd. Thus Mr. Dixon remarks, " the 'windows of a 

 building certainly have to bear an equal strain with the 

 walls, and I suppose it would be immaterial to the glass 

 whether it was placed vertically or horizontally. A 

 densely packed crowd hardly weighs £0 lbs. per square 

 foot of the space it stands upon. Reduce therefore the 

 theory to common sense ; would any one dream of 

 standing on a floor formed of glazed window sashes ? " 

 On the whole we rather think not, even if, to make 

 the case analogous, means were taken to distribute 

 the pressure uniformly, and we are forced to the con- 

 clusion that either the Bidston Observatory is a very 

 strongly constructed building with window-sashes and 

 glass of unusual strength, or that the anemometers are 

 untrustworthy. 



Leaving now the question of the maximum pressure of 

 the wind to be decided by meteorologists, there remains to 

 be ascertained what that pressure would have to reach on 

 the banks of the Thames to endanger the existence of the 

 obelisk. Mr. Dixon's assurance has probably set the fears 

 of many at rest ; he says : " As to its stability there need 

 be no fear — 130 lbs. of wind-pressvu-e would not upset it. 

 The columns of the Times are not the place to ventilate 

 calculations and figures," We can assure Mr. Dixon 

 that these calculations would be of sufficient interest tp 

 the readers of Nature to find a place in its columns, but 

 in their absence we are obliged to fall back on our own. 

 The widths of the top and bottom of larger face of the 

 obelisk are respectively 64 inches and 95 inches, the height 

 being 60 feet 6 inches exclusive of the pyramidal point, 

 which would be 7 feet high if intact ; assuming, then, an 

 additional foot of height for the lower rounded end, the 

 moment about the base of the pressure on the area of the 

 larger face will be 12,931 foot pounds for a wind pressure 

 of I lb. on the square foot. The weight of the stone is 

 estimated at 196 tons, whence, on the assumption of per- 

 fect rigidity, the ultimate stabiUty would be 196 tons X 

 radius of base (2"5), and the corresponding wind pressxu-e 



^ 196 X 2-5 X 2240 _ 84.88 lbs. per square foot. But 



12,931 

 the material of the obelisk not being perfectly rigid, it 

 will be seen that this ultimate stability could not be 

 reached. The effect of the wind-pressure is to cause a 

 deviation of the line of action of the resultant pressure 

 on the base from its centre with a diminution of the 

 stress on the windward, and an increase of that on the 

 lee-side of the base ; if the decrease exceeds the normal 

 pressure due to the weight the joint will tend to open, 

 while if the increase is carried too far it may reach the 

 crushing strength of the material. 



Both these effects have to be considered. Now in the 

 ueaign of masonry work of a substantial character it is 



c 



