October, 1910 



AMERICAN HOMES AND GARDENS 



E THE EDITOR'S NOTEBOOK 



OUR IGNORANCE OF WIND PRESSURE 



IN view of the great size of modern engineering and archi- 

 tectural structures, and the vast areas which they ex- 

 pose to the wind, it is strange that our knowledge of 

 wind pressure, both as to its amount for given velocities and 

 its action on surfaces of different form, should be compara- 

 tively limited and uncertain. The subject of wind pressure 

 is of the first importance; for in some framed structures, 

 such as bridges, roofs, etc., cases have occurred in which the 

 calculations have shown that the wind stresses have been 

 greater than those arising from the weight of the structure 

 itself or of the load which it carries. As an instance of the 

 magnitude of the pressures to which exposed surfaces are 

 liable, we refer to a large sign, 60 by 90 feet in area, 

 erected above a hotel in this city, which in a gale of wind 

 is liable to experience a maximum pressure of no less 

 than 94>^ tons. 



Our present knowledge regarding wind pressure on ex- 

 posed surfaces is largely based on certain observations by 

 the late Sir Benjamin Baker, taken prior to and during the 

 erection of the Forth Bridge, Scotland. Mr. Baker pro- 

 vided a large surface, which was so supported that not only 

 was the total pressure on the whole surface automatically 

 recorded, but, by means of pressure gages, any local excesses 

 of pressure on smaller areas of the board were separately 

 recorded. The data thus gathered established the hitherto 

 unsuspected fact that a strong wind does not blow with even 

 intensity, but is composed of masses of air moving with 

 varying velocity and exerting varying pressures. It was es- 

 tablished, or rather assumed, on this basis, that a small 

 structure such as a 100-foot county bridge was liable to be 

 exposed over its whole surface to a much higher unit pres- 

 sure than a 1,710-foot span like those composing the Forth 

 Bridge. The British Government regulations required that 

 bridge to be built for an average pressure of ^6 pounds per 

 square foot, but Sir Benjamin's experiments proved, or 

 rather suggested, that while this might be advisable for a 

 short bridge, it was too high for a bridge of great length. 

 Hence, bridge engineers are now using for long-span bridges 

 a uniform pressure of 30 to 35 pounds per square foot. In 

 the interests of safe engineering, however, it is desirable 

 that the data secured by Baker be amplified, and their truth 

 further established, by more elaborate investigations carried 

 on at different locations, and for longer periods of time. 



Another branch of this subject which calls for additional 

 investigation is the question of the pressure of the wind on 

 inclined and on curved and irregular surfaces. We all know 

 that the wind pressure is less on a cylindrical column than it 

 is on a square column of equal projected area. One experi- 

 mentalist found that the difference was in the ratio of two 



to three; but the data upon this subject also are based upon 

 a too limited range of observation. Here is a field of re- 

 search of the very highest importance, which should be at- 

 tractive to our technical colleges, and to those scientific in- 

 stitutions which have the means and the time to devote to 

 an investigation of this character. 



TO LIMIT OUR FOREST FIRES 



THE resistless sweep across some of the richest forests 

 of the Western States of this season's conflagra- 

 tions, with their frightful toll of human life and de- 

 struction of valuable timber, is appalling and altogether dis- 

 couraging. In casting about for remedial or preventive 

 measures, by which the progress of these fires can be ar- 

 rested and confined within limited areas, the most feasible 

 method would seem to be that of the provision of fire ave- 

 nues, of the same general character as those which are do- 

 ing such good service in the forest-covered sections of 

 Europe. 



For many reasons it is impossible to establish and main- 

 tain a system of forest protection as elaborate and efficient 

 as those abroad. The principal diflUculties in the way of 

 this are the vast areas to be covered, and the relatively high 

 cost of labor. The European forester makes his calling his 

 life business, knowing that if he give faithful service, there 

 is before him a life job. He is clothed with considerable 

 authority; in many cases he possesses the power of arrest; 

 and his position is strengthened by the certainty that punish- 

 ment for breach of the laws, and particularly those against 

 fire, is swift and sure. The pay, however, is so small that 

 it is possible to maintain in Europe a force which would be 

 altogether out of the question in the United States. 



Under existing conditions, it would seem that the most 

 effective way to safeguard our forests would be to cut 

 through them a network of fire avenues, following the Euro- 

 pean practice of making the width of the clearing twice the 

 height of the tallest timber through which it is cut. In 

 Europe the avenues in some forests will be found at Inter- 

 vals of half a mile; here they would be opened at Intervals 

 of from two to five miles, according to the configuration 

 and value of the forest lands. The cost of the work, if it 

 were judiciously laid out and properly executed, might be 

 covered largely, and in some localities altogether, by the 

 sale of the timber. The avenues could be so located with 

 regard to the streams and rivers or the slope of the country, 

 that the timber might be shot to the streams and floated 

 out, or hauled out by sleds when the ground was snow cov- 

 ered. If the cleared avenues were seeded, it would be pos- 

 sible to rent the grazing privileges In consideration of the 

 owners of the stock keeping down the undergrowth. 



