The first consideration in any power plant is that it shall be thoroughly reliable in operation, and the second is that it shall be economical, not only in operation, but in maintenance and depreciation. Therefore, in using the comparative efficiency curves shown in Mr. Wilson’s paper it should be kept in mind that the cost of power is not only the fuel cost, but the fuel plus the maintenance and depreciation charges, and that the latter items should not be taken from the first year’s account, but as an average of at least five years. The small gas engine is a very satisfactory apparatus when supplied with good, clean gas, and when given proper attention, but great caution should be used before investing in large units, until further developments in the art take place, as conservation of capital is just as important as conservation of coal. B. W. DUNN, Esq.[31] (by letter.)--The growing importance of investigations of explosives, with a view to increasing the consumer’s knowledge of proper methods for handling and using them, is evident when it is noted that the total production of explosives in the United States has grown from less than 9,000,000 lb. in 1840 to about 215,000,000 lb. in 1905. Table 5 has been compiled by the Bureau of Explosives of the American Railway Association. TABLE 5.--Manufacture of Explosives in the United States, 1909. ---------------------+-------------+------------------------------ Kind of explosives. | Number of | Maximum Capacity, in Pounds. | factories. +--------------+--------------- | | Daily. | Annual. ---------------------+-------------+--------------+--------------- Black powder | 49 | 1,220,150 | 366,135,000 High explosives | 37 | 1,203,935 | 361,180,500 Smokeless powders | 5 | 75,686 | 22,705,800 ---------------------+-------------+--------------+--------------- The first problem presented by this phenomenal increase relates to the safe transportation of this material from the factories to points of consumption. A package of explosives may make many journeys through densely populated centers, and rest temporarily in many widely separated storehouses before it reaches its final destination. A comprehensive view of the entire railway mileage of the United States would show at any instant about 5,000 cars partially or completely loaded with explosives. More than 1,200 storage magazines are listed by the Bureau of Explosives as sources of shipments of explosives by rail.