Mr. White's Design for the New London Bridge. 265 



1812, the specimens were tried, and the result was as in the 

 following tables. 



N. B. The figures 1, 2, 3, denote where the bars were 

 taken from; i.e. the room (1), the spout (2), or the cistern (3): 

 and those underneath denote the number of pounds required 

 to break them. 



Hence it appears that cement is hardened quicker when 

 made wet and dry than when kept quite dry, or covered with 

 water; though by the first table the bars in the room were the 

 hardest : but it would take some days for them to dry in that 

 situation, and therefore they have the same advantage as if they 

 had been placed in a spout. But the effect of a little moisture 

 is better shown in the following specimen. On the 10th of 

 October 1811, before mentioned, a bar was made as the pre- 

 ceding, and into the water used in mixing it up was put some 

 common salt. On the 15th of April, 1812, the bar was tried, and 

 broke with pounds as follows, having been kept in the wet : 



148. 118. 168. 152. = 154- mean. 

 Being in the proportion of 154 to 118 stronger than those 

 placed in the spout, which could only arise from the moisture 

 imbibed by the salt. 



The preceding experiments show the effects of time and 

 moisture (taken separately or together) on cement. 



On the same 10th of October, a bar was made in which the 

 water was 8 ounces instead of 7"3, and another with 10 ounces 

 water, both being kept in the room till the 15th of April, 1812, 

 on which day they were tried. 



Bar with 8 oz. water; 56. 56. 53. 60. 56. 53. = 55 mean. 

 Do. ... 10 oz. 36. 34. 36. 35. 30. 42. = 35i mean. 



By this it appears that a surplus of water of only 10 per 

 cent, beyond what is absolutely necessary, makes the cement 

 weaker in the jnoportion of 89 to 55. 



On the same 10th of October a bar was made, composed 

 N.S. Vol. 11. No. 64. April 1832. 2 M of 



