HEAT. 



fluids, such as are used by brewers 

 and other manufacturers, are firmly 

 bound together by strong iron hoops ; 

 these hoops, which are at first made 

 too small to fit, are heated until they 

 are sufficiently enlarged ; they are then 

 driven on, and suddenly cooled, by 

 throwing water upon them ; the con- 

 traction of the iron, which ensues on 

 cooling, brings the parts of the vessels 

 into closer contact than they could 

 easily be brought by other means, and 

 fixes the hoops firn.ly round them. 



The parts of carriage wheels are 

 bound together in a similar way ; the 

 iron band, or tire, is made a little smaller 

 than the circumference of the wooden 

 part of the wheel : being put on while 

 it is enlarged by heat, it is suddenly 

 cooled, and by its contraction binds the 

 parts of the compound wheel together 

 with great force. 



The force with which metals expand 

 when heated, and contract when cooled, 

 is capable of overcoming powerful re- 

 sistance. This may be illustrated in re- 

 irard to contraction, by an experiment 

 which succeeded some years since at 

 the Conservatoire des Arts et Metiers, 

 in Paris. The two side walls of a gallery 

 at that place, having been pressed out- 

 wards by the weight of the floors and 

 roof, M. Molard proposed making seve- 

 ral holes in the walls, opposite to each 

 other, through which strong iron bars 

 were introduced so as to cross the 

 apartment, their ends projecting out- 

 side the walls. Strong circular plates 

 of iron were screwed on to these pro- 

 jecting ends. The bars were then 

 heated, by which their ends were made 

 to project farther beyond the walls, per- 

 mitting the circular iron plates to be 

 advanced, which they were until they 

 again touched the walls. The bars, on 

 cooling, contracted, and drew the walls 

 which were receding from each other, 

 closer together. This process being seve- 

 ral times repeated, the walls were made 

 to re-assume their proper perpendicular 

 position, and might easily havj been 

 curved inwards, by the application of 

 the same means. 



The sudden expansion of bodies by 

 heat occasions some effects which re- 

 quire to be guarded against. Thus, 

 glass is very liable to break when heat 

 is applied to it, on account of the un- 

 equal expansion which is occasioned. 

 Glass being a bad conductor of heat, 

 when one surface of any vessel or plate 

 of this substance has its temperature 



suddenly raised, that surface is ex- 

 panded, but the heat not being able to 

 pass quickly through to the other sur- 

 face, that part is not at all or but very 

 little expanded, and the unequal expan- 

 sion of the two surfaces occasions the 

 glass to break. From what has been 

 said, it will appear that there is most 

 danger where the glass is very thick : 

 boiling water may be poured into a very 

 thin glass vessel without danger, be- 

 cause the heat passes through thin glass 

 in time to make both its surfaces 

 stretch equally. Looking-glasses have 

 often been broken by heating one sur- 

 face with a candle or lamp ; and elec- 

 trical-machine plates have many times 

 been destroyed by setting them before 

 a fire, one surface being expanded by 

 the heat of the fire, while the other is 

 probably contracted by a current of 

 cold air rushing towards the fire ; the 

 inequality of temperature producing in- 

 equality of expansion, occasions the 

 tlass to crack with considerable noise, 

 o cold showers of rain and warm sun - 

 shine succeeding each other, occasion 

 loss in sky-light windows. 



Other brittle substances are liable to 

 similar accidents from the same cause ; 

 heated plates of cast iron are very liable 

 to be broken by suddenly pouring 

 cold water upon them. 



The expansion and contraction oc- 

 casioned by variations of temperature 

 in the metals forming the pendulums of 

 clocks, and the balance-wheels of 

 watches, have been found to occasion 

 great irregularities in the movements of 

 these machines. The rate of going, in 

 common clocks, depends upon the 

 length of the pendulum. When the 

 pendulum is lengthened, by heat or any 

 other cause, the clock goes slower ; and 

 when it is shortened the motion is 

 quickened. The ball of a pendulum that 

 vibrates seconds, being lowered one 

 hundredth part of an inch, the clock 

 will lose ten seconds in twenty-four 

 hours. By the foregoing table of ex- 

 pansion, it may be found that a seconds 

 pendulum, the length of which is 

 39.13929 inches, will be lengthened by 

 an increase of temperature equal to 30 

 of Fahrenheit's scale, T i a th part of an 

 inch, which will occasion an error of eight 

 seconds in twenty-four hours. Various 

 contrivances have been introduced/or 

 the purpose of remedying these defects, 

 by making the expansibility of some 

 metals counteract that of others. 



The first of these was the invention of 



