248 Mr Sang on the Construction of Circular Towers, 



gible ; their strengths must necessarily have reference to the 

 material of which they are formed, to the relative thermal 

 expansions of their component parts, and to the compressibi- 

 lity and distensibility of these. When in a state of fusion all 

 the parts of such rocks are subjected to a hydrostatic pressure, 

 so that we may expect less difference of cohesive strengths in 

 various directions than is exhibited in stratified rocks. 



In general, then, we arrive at this conclusion, that stones 

 are less able to bear distensions than compressions ; that it is 

 nearly indifferent in what direction stones from massive rocks 

 may be placed, but that stones from strata ought always to be 

 so placed that the greatest pressure to which they are subject- 

 ed may be perpendicular to the plane of stratification. 



Keeping these principles in view, let us examine the ordi- 

 nary construction of signal towers. 



These towers are built in horizontal courses, gradually 

 spreading out at the base to resist the greater strain ; now 

 here it is apparent that the weight of the central part of the 

 tower must be sustained only by the foundation immediately 

 under it, unless the projecting stones be bent downwards at 

 their inner ends, so as to cause their outer extremities to bear 

 some part of the load placed on them. It is in this way alone 

 that the weight of the structure can be distributed over the 

 extended foundation when the courses are horizontal. The 

 ordinary mode of constructing such towers thus call into 

 action that species of strain which stones are least of all ca- 

 pable of resisting ; the inequalities in the mechanical states of 

 the integrant parts has leave to act, and the necessary conse- 

 quence must be a shortening of the duration of the building. 

 When such towers are subjected to the action of the waves, 

 the outer stones, which resist the first and heaviest blows, are 

 much less securely fastened than otherwise they might be, 

 and thus another imperfection makes its appearance. 



The inefficiency of the horizontal courses in preventing 

 dislocation by heavy seas is so obvious, as to have given rise 

 to the introduction of dovetails and dowalls, for the purpose 

 of locking the stones of one course, and also the different 

 courses together. The formation of dovetails occasions a great 

 waste of material, is expensive in workmanship, and, after all, 



