46 MECHANICS. 



than half this strain, or about one ton. The strength 

 increases or diminishes according to the size of the 

 cross-section of the rope ; thus a cord half an inch in 

 diameter wiU support one quarter as much as an inch, 

 and a quarter-inch cord a sixteenth as much. A knowl- 

 edge of the strength of ropes, as used by farmers in 

 windlasses, pulleys, drawing loads, &c., would some- 

 times prevent serious accidents by their breaking. The 

 following table may therefore be useful : 



Diameter of rope or Pounds borne Breaking 



cord in inches. with safety. weight. 



One eighth 31 lbs. 78 lbs. 



One fourth 125 " 314 " 



One half 500 " 1250 " 



One 2000 " 5000 " 



One and a quarter 3000 " 7500 " 



One and a half 4500 " 12,500 " 



These results will vary about one fourth with the 

 quality 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 exerted, so as to split it. The splittmg of tim- 

 ber or wood for fuel is, however, accomphshed 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 m liquids ; 

 it is sufficient, however, to give a round or spherical 

 shape to very small portions or single drops, and to fur- 

 nish a beautiful illustration, on a minute scale, of the 

 same principle which gives a rounded form to the sur- 

 face of the sea. In one case, cohesion, by drawing to- 

 ward a common centre, forms the minute globule of 



