20O Thickness, Cement and Materials of Walk. 



the breach in the wall, on the outside. We put an end 

 to these nocturnal sorties, by directing the barrack mas- 

 ter, to cause a lai'ge trench to be dug along the walls, 

 outside ; and whariF logs to be laid therein, from the 

 foundations, to the surface of the ground. This was 

 eifectual; though the masonry opposed a feeble barrier 

 against the efforts of these involuntaiy tenants of this 

 dreaiy mansion. This instance strongly proves the 

 mistake I have mentioned, made by even intelligent 

 builders. This goal was built by Robert Smith, and 

 John Palmer. The one the most celebrated architect, 

 and the other among the best masons and bricklayers, 

 of that day. I do not recollect the thickness of the 

 walls, but believe they exceed three feet, in the groimd 

 story. The stones in general were not sufficiently large, 

 though many were so; the mortal' was too rich, and the 

 wall unnecessarih' thick. Nor were there a sufficient 

 number of ashlars of size, on the faces; or headers, run- 

 ning through the walls, transversely. To guard against 

 such defects, the materials must be sound, and the 

 stones reasonably large, for every part of thick walls. 

 So much time has elapsed since the lacts relating to the 

 rhiladelphia prison- occun-ed, that the mortar has un- 

 questionably indurated; and the building become com- 

 petent to all its purposes. 



The hardest stone is not the most eligible : softer 

 stones, easier worked under the hammer, or chissel, and 

 of clean grit, are by far the best, They indurate suffi- 

 ciently in the air or sun, are tenacious of the cement, 

 and absorb and conduct the damp vapour, instead of 

 repelling it, as do hard stone, by which constant mois- 

 ture is retained on the walls. When this moisture 



