of lime and sulphate of lime, or gypsum. The moderate use of soda ash (say one part in 5,000 of water) holds this deposit in check, by producing from the principal ingredient a neutral carbonate of lime, which will not adhere to the plates, when thus rapidly formed. Soda ash, if used in excess, boils up and passes into the cylinders and pumps, clogging up valves and pistons by combining with the lubricants. If the gauge-glasses become muddy, too much soda water is used. It is much better to supply the boilers with pure water that can deposit no scale, this being done by means of filters and heaters, or by surface-condensers, and being especially advisable with sectional and water tube boilers. HOW TO PREVENT ACCIDENTS TO BOILERS. Ist. Carry regular steam pressure. 2d. Start the engine slowly, so as not to make a violent change in the condition of the water and steam, and when consistent stop the engine gradually. 3d. Carry sufficient water in the boiler. 4th. Do not exceed the pressure in lbs. per square inch allowed to be carried. 5th. See that every appliance of the boiler— feed pipes and safety-valve, fusible plugs, etc.— are in complete working order. WEIGHT OF DIFFERENT SUBSTANCES. The following table shows the weight in pounds of a cubic foot of various common substances. From this table we can calculate the weight of any mass of any substance in the table, by first ascertaining by measurement and calculation how many cubic feet there are in the mass, and multiplying the result by the weight of a cubic foot of the substance: The following table shows the breaking strength of, or power in pounds required to pull apart a bar one inch square of various common substances. The strength of any piece of substance shown in the table can be found by measuring or calculating the number of square inches area of the piece and multiplying the result by the breaking strength of one square inch, as shown in the table: SUBSTANCE. Wrought copper Cast iron-American Cast iron-English . English bar Boiler plate. Platinum wire Cast silver. Bar cast steel Shear steel. Cast tin Zinc, cast HARDNESS OF SUBSTANCES. It is, of course, impossible to speak of hardness in pounds, or square feet, or anything of that kind. We can only compare the hardness of one substance with the hardness of another substance. Thus, we can say that chalk is half as hard as gold. It is best to take some one substance as a standard and compare other substances with it. Taking Ormuz diamond as the standard, we find cork to be one-twentieth as |