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76                         METALLURGY   OF   CAST   IRON.
Lowthian Bell claims that where this is done all the economy is achieved that is practical to be expected in making iron, as long as the present fuel is used. To note the manner in which heat is produced, absorbed and lost, see Tables 15, 16 and 17, page 72.
Pyrometers. Various methods are employed for measuring degrees of heat. Those of a crude nature consist, for example, in using dry sticks of wood, which when inserted in hot air take fire, indicating a temperature of about 650 degrees F. Again, sticks of zinc, if melted, indicate about 750 degrees. To obtain a record of higher temperatures in a more accurate manner, many different kinds of instruments have been devised and in recent years have been largely adopted. A pyrometer recently designed and patented by Mr. E. A. Uehling, of Birmingham, Ala., in which the expansion and contraction of air between two small apertures is the principle used to denote temperature, is claimed to be giving excellent satisfaction. It is being largely adopted by blast furnacemen to record for them any variations in the temperatures of the hot blast or escaping gases, and enables them to regulate the workings of a furnace so as to give a greater output and produce a more uniform product than heretofore.
The question of temperatures in driving a furnace fast or slow is one of interest. It will appear strange to the founder, as well as to others, that a furnace can be got so il hot " as to retard the speed of making iron, and also may result in li scaffolding; " nevertheless there is a limit to attaining temperatures best calculated to drive a furnace to its utmost, which means obtaining the largest tonnage possible in making iron.cels all                |j