20 Mr. White’s Design for the New London Bridge,

Suppose your cement bricks as strong as the mean of these,or say it would require lOOOlbs. to break it. Then a brick of70 feet span (supported at that distance) would be 110 timesthe extent of the common bricks above, and would be110x110 = 12,100 times as strong, or would require tobreak it 12,100 x 1000 = 12,100,000lbs.

The weight of that part of the experiment brick interposedbetween the two supports would weigh about 6lbs.; of coursethe weight of one 110 times the lineal dimensions would be110 x 110x100 times 6lbs. or 1,331,000 x 6 = 7,986,OOOlbs.say 8,000,OOOlbs. Half this, or 4,000,000 only, would haveau action on the brick to break it, consequently it would carry8,000,000 besides itself.

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

besides its own weight.

If a brick were 8 feet deep, its strength would

be £th of. 12,000,000 = 1,333,333

and its acting weight ^rd of 4,000,000 = 1,333,333leaving no weight to carry, or it would just carry itself.

But such a brick would carry more than itself, inasmuchas the part ( a a) which would be beyond the supports wouldhelp to balance the middle part ( b ) : —a circumstance not

considered in the ex-- : ---—

periment brick, and in- ' " : " . .. : a I

deed too trifling to af-fect the conclusion here

drawn. So that a brick . ' eet .

82£ feet long

38 — wide '

8 — deep, of the same strength of materials as

the experimental bricks, would carry something more than itsown weight, but would weigh 1340 tons.

Yours truly, John Southern.

N.B. The common bricks were about 6| inches long, 4^broad, and 2f deep.

VIII. On