071 the Principle of Cementitioiis Architecture. 269 



This experiment was made before Mr. Jessop and Mr. 

 Southern. 



Dear Sir, Oakhill, August 27, 1811. 



I yesterday made three experiments on the strength of 

 common bricks. They were made at Heath, and pretty well 

 burnt. 



In the experiments the supports were 7f ths asunder, and 

 the pressure was applied in the middle. 



In the first experiment the brick was broken, after waiting 



a short time, by 930 lbs. 



In the second, by 977 



In the third it sustained 1095lbs. for some "J 

 minutes without breaking, but broke almost V1313 

 immediately with 1313lbs J 



3)3220 



Mean 1073 



Suppose your cement bricks as strong as the mean of these, 

 or say it would require lOOOlbs. to break it. Then a brick of 

 70 feet span (supported at that distance) would be 110 times 

 the extent of the common bricks above, and would be 

 110x110 = 12,100 times as strong, or would require to 

 break it 12,100 x 1000 = 12,100,000lbs. 



The weight of that part of the experiment brick interposed 

 between the two supports would weigh about 6lbs. ; of course 

 the weight of one 110 times the lineal dimensions would be 

 110X110X100 times 6lbs. or 1,331,000 x 6 = 7,986,000lbs. 

 say 8,000,000lbs. Half this, or 4,000,000 only, would have 

 an action on the brick to break it, consequently it would carry 

 8,000,000 besides itself 



This brick would be 38 feet wide and 24 feet deep. If a 

 brick of the same span and width (70X38), but half the depth, 

 were made, it would be only \ the strength whilst it would be 

 ^ the weight; or 



Its strength would be 3,000,000 



Its acting weight 2,000,000 



So that it would bear 1,000,000 



bsidts its n weight. 

 If a brick were 8 teet deep, its strength would 



be ^th of 12,000,000 = 1,333,333 



and its acting weight ^rd of 4,000,000 = 1,333,333 

 leaving no weight to carry, or it would just carry itself 



But such a brick would carry more than itself, inasmuch 

 as the part [a a) which would be beyond the supports would 



help 



