ABRASION OF THE SURFACES OF SOLIDS. 155 



REMARKS.— From the foregoing experiments it appears that there is a great deal of irregularity 

 in the results. 



Increase of pressure scarcely increasing the resistance. This may arise in some degree from the 

 surfaces becoming condensed, and thus rendered less liable to abrasion. In some of the cases abra- 

 sion had already commenced, but it was not convenient to pursue the experiment further. 



The soft woods present more resistance than the hard woods. 



Yellow deal on yellow deal being the greatest. 



Red teak on red teak the least. 



According to Mr. Knowles of the Navy Office, F.R.S., the weight of the Prince Regent of 120 

 guns on the slips previous to launching, was 2400 tons ; which, divided by the area of the sliding 

 surface of her bilge-ways (equal to 149,184 square inches), gives a pressure of 36lbs. per square inch. 



But the weight of the Salisbury of 58 guns on the slips, according to the area of her bilge-ways, 

 was 441bs. per square inch. Now, by the foregoing Table, the average force required to put in 

 motion the three different kinds of oak, under a pressure of 56lbs. per inch, is about |th of the 

 pressure, which proportion prevails even as high as 6cwt. per inch area : and by Table IX. we find 

 tliat soft soap (the ingredient mostly used for diminishing the friction of bilge-ways under a pressure 

 of 561bs per inch,) gives about j'^th of the pressure for the friction. Hence the angle at which a build- 

 ing slip should be laid can be easily determined. Coulomb even makes 49lbs. per square inch, and 

 g-^th for the pressure for hogslard. 



The weight of the middle arch (of 151 feet 9 inches span) of the New London Bridge, together 

 with the centres, is 4900 tons. This acting upon the surface of the striking wedges equal to 540 

 square feet, gives a pressure of 140lbs. per square inch. The angles of inclination of the wedges are 

 equal to 8° 45', and their surfaces are covered with sheets of copper well coated with tallow. On 

 removing the check pieces, the wedges commenced gliding back slowly and uniformly by the gravity 

 of the arch and centres, and the motion was checked and continued until the arch was left in 

 equilibrio. 



Plate V. 



This apparatus was constructed both for brass and iron. The pivots were 

 accurately turned, and the suspending slings loosely hung. The total space 

 passed over did not exceed four inches and a half. The cord was of the best 

 sash-line, and the pulley very sensible. The rigidity of the former and friction 

 of the latter were accurately ascertained, by trials at different weights. The 

 block was of cast iron accurately bored. The axle was allowed to have full 

 play in the block, in order that no binding might take place. The space 

 passed through was denoted by marks on the axle and block. The time by a 

 seconds watch. 



An improvement was afterwards made in the apparatus, by substituting a 

 roller of cast iron working in a block, and having a cord wound round its 

 surface so as to allow of a descent of the moveable weight of 21 feet. 



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