266 Mr. White's Design for the Xc-v London Bridge, 



of three measures of cement and two measures of sand, mixed 

 with an unknown quantity of water, but which was stiff enough 

 to make the mass moderately plastic. On the 15th of April 

 it was tried, and broke with pounds as follows: 



Bar with three measures cement and two sand : 

 74. 90. 91. 76. 50. = 76 mean. 



On the same 10th of October was made a bar of two mea- 

 sures cement and two measures brick-dust, which, tried on 

 the 15th of April, broke with pounds as follows: 



43. 45. 50. 59. 58. = 52 mean. 

 These two last experiments cannot be compared together, as 

 the cement was not in the same quantity in each ; but it is likely 

 that brick-dust is not so fit for mixing with it as sand. 



Besides the bars used, others are in reserve for future trials. 



Bric/cs-^Some bricks were made in July 1811, of three 

 measures cement and two measures sand, and were broken, 

 27th of January 1812, by a weight placed on the middle, the 

 supports being 8 inches asunder ; 6 of them were thus broken 

 separately, and the following were the number of pounds re- 

 quired to do it : 



454. 514. 584. 544. 520. 524. = 522 mean. 

 At the same time as the above were made and tried, as many 

 bricks composed of three measures of mineral cement and 

 two of sand, which broke with pounds as follows: 



534. 589. 409. 518. 442. 534. = 505 mean. 



At the same time were made some bricks of Koman cement 

 only, but which were not tried till March 1812, being three 

 months older than the preceding, and broke with pounds as 

 follows : 



536. 536. 456. 501. 470. = 500 mean. 



Thus it appears that these latter are not so strong as if sand 

 had been used in them, which is contrary to the results obtained 

 from the bars ; but in the cement bricks were a number of very 

 fine cracks, and which did not happen in those where sand 

 was used. Otherwise the cement is much harder when used 

 by itself; but the cracks can hardly be avoided in large masses, 

 and therefore are made in a brick, though not in a bar, of 

 the foregoing dimensions. 



N.B. These bricks were 4 inches wide and 2^ thick. 



Next was tried the strength of common bricks, the sup- 

 ports being 7 inches asunder instead of 8. 



1st. Five of the soundest paving bricks were selected, which 

 broke with the following pounds: 



1254. 784. 332. 761. 890. = 804 mean. 



2ndly, 



