STRENGTH OF MATERIALS. 31 



These results will vary about one-fourth with the quaV 

 ity of common hemp. Manilla is about one-half as strong 

 as the best hemp. The latter stretches one-fifth to one- 

 seventh before breaking. 



Wood is about seven to twenty times stronger when 

 taken lengthwise with the fibres than when a side force is 

 eixerted, so as to split it. The splitting of timber or wood 

 for fuel is, however, accomplished with a comparatively 

 small power by the use of wedges, the force of heavy 

 blows, and the leverage of the two parts. 



The attraction of cohesion is very weak in liquids ; it is 

 sufiicient, however, to give a round or spherical shape to 

 very small portions or single drops, and to furnish a 

 beautiful illustration, on a minute scale, of the same prin- 

 ciple which gives a rounded form to the surface of the 

 sea. In one ctise, cohesion, by drawing toward a common 

 centre, forms the minute globule of dew upon the blade 

 of grass ; in the other, gravitation, acting in like manner, 

 but at vast distances, gives the mighty rotundity to the 

 rolling waters of the ocean. 



CAPILLARY ATTRACTION. 



Capillary attraction is a species of cohesion ; it takes 

 place only between solids and liquids. It is this which 

 holds the moisture on the surface of a wet body, and which 

 prevents the water from running instantly out of a wet 

 cloth or sponge. By touching the lower extremity of a 

 lump of sugar to the surface of water in a vessel, capillary 

 attraction will cause the water to rise among its granules 

 and moisten the whole lump. It may be very distinctly 

 shown by placing the end of a fine glass tube into water ; 

 the water will rise in it above the level of the surrounding 

 surface. If the bore of the tube be the twelfth of an inch 



