TRANSACTIONS OP THE SECTIONS. 



243 



Tenacity in lbs. Ultimate extension in 



per sq. inch. decimals of length. 



Staffordshire: strongest 62231 0*222 



weakest 56715 0*225 



West of Scotlaud : strongest 64795 0-173 



„ weakest 56655 0-191 



Sweden: strongest 48232 0-264 



weakest 47855 0*278 



Russia: strongest 56805 0*153 



weakest 49564 0-133 



Iron Plates. 



Tenacity in lbs. Ultimate extension in 



per sq. inch. decimals of length. 



Yorkshire: strongest lengthwise 56005 - 141 



„ weakest lengthwise 52000 0*131 



„ strongest crosswise 50515 0*093 



„ weakest crosswise 46221 0*076 



Note. — The strongest lengthwise is the weakest crosswise, and vice versa. 



Steel Bars. 



Tenacity in lbs. Ultimate extension in 



per sq. inch. decimals of length. 



Steel for tools, rivets, &c. : strongest 132909 0*054 



weakest 101151 0*108 



Steel for other purposes : strongest 92015 0*153 



„ weakest 71486 



Steel Plates. 



Tenacity in lbs. Ultimate extension in 



per sq. inch. decimals of length. 



Strongest lengthwise 94289 0*0571 



Weakest lengthwise 75594 0*1982 



Strongest crosswise 96308 0*0964 



Weakest crosswise 69016 0*1964 



Note. — The strongest and weakest lengthwise are also respectively the strongest 



and weakest crosswise. 



On the Comparative Value of Propellers. 2?y John Robb. 



Robertson's Patent Chain Propeller. By Peter Spence. 



The peculiar principle of Mr. Robertson's invention is, that he applies the power 

 by dragging the vessel from a fixed point ; and its great ingenuity is, that the fixed 

 point is at the same time a moveable one, a constantly fixed point in relation to the 

 power exerted by the engine in propelling the vessel, and a constantly changing 

 point in relation to the course ou which the vessel is being propelled. The con- 

 struction of the propelling apparatus is as follows : — At or near the bows of a boat, 

 say 70 feet long, is placed a steam-engine, the main shaft of which crosses the 

 bows of the vessel at or about the level of the deck ; a fixed pulley is attached to 

 each end of this shaft, these pulleys projecting over the sides of the vessel ; they 

 are three feet or more in diameter, and on their periphery have a hollow or 

 groove to receive the chains which are to run over them ; they are also so constructed 

 as to take a firm hold of the chains as the power is exerted in dragging the chains 

 over the pulleys. On the other or the stern end of the boat are two pulleys, also 

 projected over, one on each side ; these are loose, so that the chains merely run over 

 them. Friction rollers are also placed along each side of the vessel, to carry the 

 chains as they pass from the stern to the bows of the vessel ; the chains, which are 

 endless, pass or are dragged over fixed pulleys at the bow of the vessel ; and falling 

 down, lie along the bottom of the canal, and thus become the fixed point or lineal 

 anchor on which the power acts ; the action of the engine in dragging the chain 

 over the loose and fixed pulleys being necessarily to drag or propel the boat forward. 



16* 



