16 Mr. White's Design for the New London Bridge,

Hence it appears tliat cement is hardened quicker whenmade wet and dry than when kept quite dry, or covered withwater; though by the first table the bars in the room were thehardest: but it would take some days for them to dry in thatsituation, and therefore they have the same advantage as if theyhad been placed in a spout. But the effect of a little moistureis better shown in the following specimen. On the 10th ofOctober 1811, before mentioned, a bar was made in the samemanner as the preceding, and into the water used in mixingit up was put some common salt. On the 15th of April, 1812,the bar was tried, and broke with pounds as follows, havingbeen kept in the wet:

148. 118. 168. 152. = 151 mean.

Being in the proportion of 154 to 118 stronger than thoseplaced in the spout, which could only arise from the moistureimbibed by the salt.

The preceding experiments show the effects of time andmoisture (taken separately or together) on cement.

On the same 10th of October, a bar was made in which thewater was 8 ounces instead of 7’3, and another with 10 ounceswater, both being kept in the room till the 15 th 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. = 35| mean.

By this it appears that a surplus of water of only 10 percent, beyond what is absolutely necessary, makes the cementweaker in the proportion of 89 to 55.

On the same 10th of October a bar was made, composedof three measures of cement and two measures of sand, mixedwith uu unknown quantity of water, but which was still'enoughto make the mass moderately plastic. On the 15th of Aprilit 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 tw r o mea-sures cement and two measures brick-dust, which, tried onthe 15th of April, broke with pounds as follows:

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

These two last experiments cannot be compared together, asthe cement was not in the same quantity in each; but it is likelythat brick-dust is not so fit for mixing with it as sand.

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

Bricks — Some bricks were made in July 1811, of threemeasures cement and two measures sand, and were broken,27th of January 1812, by a weight placed on the middle, the

supports