. Journal. the presentquestion, viz. that of comparing the effect of wind on cylinderswith that on plane surfaces. The conclusion the author comes to on this point is, thatinasmuch as results of experiments give coefficients of *57 and•74, whereas theory gives 67, this latter may be looked uponas a fair average value, if indeed the higher coefficient 74should not be adopted as being on the safe side. Neverthelessthe coefficient universally adopted in practice and sanctionedby Rankine, probably on account of Bordas experiments, is• 50. This is nearly as much lower than Bordas coefficient asBord
. Journal. the presentquestion, viz. that of comparing the effect of wind on cylinderswith that on plane surfaces. The conclusion the author comes to on this point is, thatinasmuch as results of experiments give coefficients of *57 and•74, whereas theory gives 67, this latter may be looked uponas a fair average value, if indeed the higher coefficient 74should not be adopted as being on the safe side. Neverthelessthe coefficient universally adopted in practice and sanctionedby Rankine, probably on account of Bordas experiments, is• 50. This is nearly as much lower than Bordas coefficient asBordas is lower than the theoretical, and seems to the authorunwarrantably far on the unsafe side. The author considers that the law of the squares of thesines agrees sufficiently with experiments to warrant him inadopting it, for the purposes of comparison between differentforms of shafts, effects of different directions of the wind, & may seem that in the absence of any reliable data as to the 156. STABILITY OF CHIMNEY SHAFTS. 157 pressure of the wind in the most simple cases, it is absurd torefine so much; but very probably the best data we shall geton this important subject of wind pressure, will be by comparingthe failure of just such structures as tall shafts, and to comparesuch failures we must have a sound basis to work upon. The force P of the wind has been taken to act horizontallybecause that will give the maximum effect. Its action wouldin reality be normal to the batter, but the batter of chimneyshafts is so small that the difference would be inappreciable. Referring to Fig. J, by taking moments we have asW = p M, where M = moment of P round the point x with pressureunity. Now if the external dimensions are fixed so that M does notvary, W will be a minimum (i. e. the least material will berequired, and therefore the least cost incurred) when x is aslarge as possible. But x is limited by the conditions I. and satisfy condition II., x must not be greater th
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