MECHANICS AND USEFUL ARTS. 57 



science to improvements in the manufacture. There appears, in fact, to be 

 no rule in existence calculated to attain uniformity of strength in all the 

 parts of a steam boiler, where some of the parts are exposed to internal and 

 some others to external pressure. 



The resistance of cylinders, spheres, etc., to internal pressure, have been 

 ascertained from experimental data, such as the form and dimensions of the 

 vessel united to the resisting powers of the material; but we have yet to 

 learn what proportion cylindrical and elliptical tubes bear to each other in 

 their resistance to external and internal pressure. 



To supply this want, a series of experiments have recently been instituted, 

 at the joint request of the British Association and the Royal Society, by Mr. 

 Fuirbairn, the eminent English engineer, and the law of resistance, under 

 various forms and conditions, ascertained. 



The law of resistance for cylindrical tubes, as regards their length, appears 

 to be this : a tube having the same strength of material, and being of the 

 same diameter, will resist double the pressure to one of double the length ; 

 or the collapsing pressure, other things being the same, varies inversely as 

 the length, and inversely as their diameters. Experiments made with 

 elliptical tubes showed that in every construction where tubes have to sustain 

 an uniform external pressure, the cylindrical is the only form to be relied 

 upon, and that any departure from the true circle is attended with danger. 



The experiments also tended to confirm the conclusions arrived at some 

 years since, that the strengths of riveted joints of malleable iron plates are 

 nearly as the numbers 



100 for the plate, 

 70 for double riveted joints, 

 50 for single riveted joints. 



In conclusion, Mr. Fairbairn remarks : It is interesting to observe how 

 closely nature approximates in her productions to the strongest and best 

 forms. If we look at the tubular forms of grasses, bamboos, and other 

 vegetable constructions of this kind, and taking to account the uses for 

 which they were intended, we shall see that the form contributes greatly to 

 their strength ; and we shall, moreover, find that the shoots are telescopic, 

 forming a series of concentric rings, arising from the formation of new and 

 smaller tubes as they emerge in succession from those previously formed. 

 As these again protrude and advance in growth, they leave behind enlarged 

 hoops, or disks, of sufficient rigidity to support and sustain the form of the 

 tubular structure. The same law which pervades natural productions should 

 not be overlooked in art. We have ever before us the lessons of this first 

 great natural teacher; and did we but consult her laws, and in all our appli- 

 cations endeavor to conform to the rules of a philosophy which never errs, and 

 by which nothing is ever made in vain, we should find, to use the words of 

 our aspirations after truth, that Nature's laws, and the constructions derived 

 therefrom, constitute the only true system of philosophy by which we can 

 attain the maximum of strength with the minimum of material. 



The sphere is probably the only true form by which we can obtain uni- 

 formity of resistance to an uniform pressure, whether external or internal ; 

 and to approximate to this, probably, was the reason why our predecessors, 

 from the days of the Marquis of Worcester to those of Watt, adopted the 

 haycock or circular boiler with a hemispherical top and hemispherical bot- 

 tom, as shown. 



