|---------------------------+-------+-------+---------| | | Fahr. | Cent. | Réaumur.| |---------------------------+-------+-------+---------| | Boiling point | 212 | 100 | 80 | | | 194 | 90 | 72 | | | 176 | 80 | 64 | | | 158 | 70 | 56 | | | 140 | 60 | 48 | | | 122 | 50 | 40 | | | 104 | 40 | 32 | | | 86 | 30 | 24 | | | 68 | 20 | 16 | | | 50 | 10 | 8 | | Freezing point of water | 32 | 0 | 0 | | | 14 | -10 | -8 | | | -4 | -20 | -16 | | Freezing point of Mercury | -40 | -40 | -32 | |---------------------------+-------+-------+---------| Alcohol is used in thermometers in very cold districts, as it does not freeze even at a temperature of -132° Fahr. We have now explained the way in which we can measure heat by the expansion of mercury in a tube. We can also find out that solids and gases expand also. Engineers always make allowances for the effects of winter and summer weather when building bridges; in summer the bridge gets longer, and unless due provision were made it would become strained and weakened. So there are compensating girders, and the structure remains safe. The effects of expansion by heat are very great and very destructive at times. Instances of boilers bursting will occur to every reader. It is very important to be able to ascertain the extent to which solid bodies will expand. Such calculations have been made, and are in daily use. We can crack a tumbler by pouring hot water into it, or by placing it on the “hob.” A few minutes’ consideration will assure us that the lower particles of the glass expanded before the rest, and cracked our tumbler. A gradual heating, particularly if the glass be thin, will ensure safety. Thick glass will crack sooner than thin. Again, many people at railway stations have asked us, “Why don’t they join the rails together on this line?” We reply that if every length of rail were tightly fixed against its neighbour, the whole railway would be displaced. The iron expands and joins up close in hot weather. In wet weather, also, the wooden pegs and the sleepers swell with moisture, and get tightened up. Everyone knows how much more smoothly a train travels in warm, wet weather. This is due to the expansion of the iron and the swelling of the sleepers and pegs in the “chairs.” A railway 400 miles long expands 338 yards in summer,—that is the difference in length between the laid railroad in summer and in winter.