the Thermal Expansion (>/' Marble. 71 



white varieties, are accustomed to guard against its effects in the 

 construction of chinniey-pieces. 



In estimating the fitness of any substance for the construc- 

 tion of clock pendulums, other considerations than that of its 

 thermal expansion must be taken. The variable buoyancy of 

 the air, and the changeable resistance which it offers to a mov- 

 ing l)ody, must also be attended to ; and it is evident that, in 

 regard to both of these, the dense has the advantage over the 

 rare material. Taking the specific gravity of marble at 2.7,* 

 it is 2268 times heavier than air ; so that a variation of one inch 

 in the barometer will make a change in the length of a beat of 

 the tjb'oou^'^ part, that is, of |ths of a second per day. 



Estimating the expansion of white marble at .001, each de- 

 gree of the Fahrenheit thermometer will cause a change in the 

 clock's daily rate of ^ of a second ; so that the common deal- 

 rod pendulum, with a leaden bob, must, both from its smaller 

 elongation, and from the diminished hydrostatic influence and 

 resistance of the air, be superior to the marble one. 



We cannot conclude a paper on the expansion of such bodies, 

 without pointing to the various expansibilities of the materials 

 used in building. It would, indeed, be a useful and an inter- 

 esting research to inquire into the actions of heat and moisture 

 upon the materials of houses ; since, to vai'ieties in these, more 

 perhaps than to chemical decomposition, the gradual dismem- 

 berment of edifices may be due. On marbles the effect of mois- 

 ture in producing expansion was imperceptible, and from some 

 incidental experiments, the Carrara marble was found to absorb 

 only about the 1800th part of its weight of water. However 

 small the expansions of such bodies may be, they are yet ac- 

 companied by enormous force, to prevent the effects of which 

 it is in vain to heap together strength and matter; since just in 

 proportion to the additional strength is increased also the de- 

 structive force. 



EuiXBUKGii, 30//i March 1831. 



" The white was 2.(jj, aiul the black 3.0. 



