THE EFFECTS OF HEAT. 



229 



"What applica- 

 tions of the 

 expansion of 

 Bolids by heat 

 arc made in 

 the arts? 



._-J^i 



j^T-^qiggcgwcra 



With what de- 

 gree of force 

 do bodies ex- 

 pand and con- 

 tract ? 



flame of an alcohol lamp, it will become so FiG. 202. 



for expanded by heat as no longer to pass 

 through the ring. 



The expansion of solids 



by heat is made applicable 



for many useful purposes in 



the arts. The tires of wheels, 



and hoops surrounding 

 ■water- vats, barrels, etc., are made in the 

 f rst instance somewhat smaller than the 

 frame- work they are intended to surround. 

 Tliey are then heated red hot and put on 

 in an expanded condition ; on cooling, they 



contract and bind together the several parts with a greater force than could 

 be conveniently applied by any mechanical means. In hke manner, in con- 

 structing steam-boilers, the rivets are fastened while hot, in order that they 

 may, by subsequent contraction, fasten the plates together more firmly. 



525. The force with which bodies expand 

 and contract under the influence of the in- 

 crease or diminution of heat, is apparently- 

 irresistible, and is recognized as one of the 

 greatest forces in nature. 



The amount of force with which a solid body wUl expand or contract is 

 efiual to that which would be required to compress it througli a space equal 

 to its expansion, and to that which would be required to stretch it through a 

 space equal to its contraction. Thus, if a pillar of metal one hundred inches 

 in height, being raised in temperature, is augmented in height by a quarter 

 of an inch, the force with which such increase of height is produced is equal 

 to a weight which being placed upon the top of the pillar would compress it 

 so as to diminish its height by a quarter of an inch. 



In the same manner, if a rod of metal, one hundred inches in length, be 

 cofttracted by diminished temperature, so as to render its length a quarter of 

 an inch less, the force with which this contraction takes place is equal to that 

 which being applied to stretch it would cause its length to be increased by a 

 quarter of an inch. 



This principle is sometimes practically applied when great mechanical forc« 

 is required to be exerted through small spaces. Thus walls of buildings 

 •which, from a subsidence of the foundation, or an unequal press\u"e, have been 

 thrown out of their perpendicular position, and are in danger of falling, may 

 be restored in the following manner : A series of iron rods are carried across 

 the budding, passing through holes in the walls, and secured by nuts on the 

 outside. The rods are then heated by lamps until they expand, thereby 

 causing their ends to project beyond the building. The nuts with which 

 these extremities are provided are then screwed up until they are in close 

 contact with the outside wall, the lamps arc then withdrawn and the roda 



