that in this analysis the coal was merely dried, and the loss in weight set down as water, while the chemically-combined water, passing off in the subsequent distillation, was reckoned with the hydro-carbon. The error, if such it is, is a vital one. The water in lignites not only decreases the amount of actual fuel, but by evaporation absorbs heat in the furnace; and it may be consequently difficult, or even impossible, to maintain high smelting temperatures with such fuel economically.

Some experiments already made have resulted both ways; but the favorable results, so far as I can learn, were obtained on too small a scale to be perfectly satisfactory, while the unfavorable ones may possibly be due to the employment of the ordinary grates and fire-bridges used for wood, which are, of course, somewhat unsuitable. Decisive tests have yet to be made; meanwhile, I am inclined to believe that the coal can be used successfully in gas-furnaces with regenerators, and perhaps not otherwise. One thing is certain, it is excellent for all domestic purposes, and for the generation of steam; and I hope that it may soon be furnished so cheaply as to supersede wood for these applications. This should make the supply of wood and charcoal for furnaces last much longer than it will at the present rate of consumption. However, it should be added that there is no lack of wood in the Rocky Mountains. The trouble is that it speedily thins out in the neighborhood of towns and metallurgical works; and the prices of labor and hauling are such as to make it expensive when brought from a distance. I hardly think, nevertheless, that the prices of fuel will rise beyond present figures at this place for some time to come. I believe Professor Hill, at Black Hawk, pays from $5 to $7 per cord for wood, and say 13 to 15 cents per bushel for charcoal.

CHAPTER XII.

THE SPEED OF STAMPS IN COLORADO AND ELSEWHERE.

The question, what is the best proportion among weight, fall, and speed of stamps, is one which has not yet received thorough and systematic examination. In considering the economical application of stamping-machinery, we meet, at the beginning, with serious difficulties in obtaining accurate data for comparison. The weight and fall of stamps vary as the shoes and dies wear out; and this may lead to a change of speed also. Moreover, defects in engines, boilers, or machinery for the transmission of power, may occasion serious losses, which cannot fairly be charged to the arrangements of the stamps proper. Again, the capacity of stamp-mills is directly dependent, in some degree, upon the nature and extent of discharge, fineness of screens, and other peculiarities of the battery. Finally, the hardness and tenacity of the rock crushed varies so much that comparisons between different localities cannot be implicitly trusted. The safest experiments are those made in the same mill, by changing first one and then another condition of working; but this is seldom possible for such conditions as weight and lift of stamps, and only within narrow limits for their speed.

We may eliminate questions of friction, transmission, and generation of power, in the case of stamps, by measuring the power actually developed by their fall. Thus, the weight, multiplied into the fall in feet, and the number of drops per minute, gives us exactly the number of foot-pounds exerted by each stamp. Dividing by 33,000, the number of foot-pounds per minute in one-horse power, we have the horse-power per stamp, from which the effective power of the whole mill may be obtained. Dividing the amount of rock crushed daily by the effective horse-power, gives us the daily amount per horse-power; and this is the best measure that can be obtained for the effectiveness of the stamps. A complete discussion of the subject would require us to determine the exact influence of the discharge, etc., and the exact resistance offered by different classes of rocks, for both of which points the data are wanting. Professor J. D. Hague, in the third volume of the United States Geological Exploration of the Fortieth Parallel, gives a valuable table of the operations of a number of mills in Gilpin County, Colorado. The discussion of this table leads to some interesting results, which I shall briefly set forth. I give a portion of it, rearranged to suit the object in view, and furnished with additional columns.

Relative efficiency of certain stamp-mills in Gilpin County, Colorado.

I have taken from the report the names of mills, number of stamps running, weight of stamps, fall in inches, number of drops per minute, and tons of ore crushed per day. To these columns I have added one giving the total horse-power developed and one giving the tons of ore crushed daily per horse-power developed. These figures are obtained by separate calculations for each mill. At the bottom of the table certain totals and averages have been added. The total number of stamps explains itself. The total weight is arrived at by multiplying the number and weight for each mill, and then aggregating these products. The total horse-power, again, is a simple addition. The methods of obtaining averages require more detailed comment. In several columns the numerical differs decidedly from the dynamical average; thus, if we multiply the number of stamps in each mill by their fall, add these products, and divide the sum by the total number of stamps, we obtain a numerical average of the fall; and a similar process gives us a numerical average of the number of drops per minute; but if we should attempt to deduce from the total number of stamps, their average weight and (numerical) average fall and speed, the total horse-power developed, we should obtain a result different from that which is arrived at by simply

* Estimated, generally from maxima and minima given. Thus 15 to 20 is put at 17).

adding the totals given in the column of horse-power developed. The reason is obvious. In taking a merely numerical average we leave out of account the weight of the different stamps; it is therefore necessary to multiply the number and weight of stamps of each mill into the drop and to divide the sum of these products by the aggregate weight of all the stamps of all the mills. In calculating the average speed the drop, as well as the number and weight, must be included. This can be best illustrated by an example, comprising, for the sake of simplicity, only two mills. I take, almost at random, Nos. 2 and 11 from the table, viz: Black Hawk: 60 stamps, 850 pounds, 14 inches, 15 drops, 27 horsepower.

Bates: 8 stamps, 425 pounds, 12 inches, 30 drops, 3.1 horse-power. The totals would be 68 stamps, 54,400 pounds, and 30.1 horse-power. The numerical averages are obtained as follows:

31.68 horse-power, whereas the aggregate horse-power, as we know by calculating it separately for each mill, is 30.1 horse-power.

The dynamical averages, on the other hand, are obtained as follows: Fall.-60 x 850=51,000 51,000 × 14=714,000 3,400 × 12 = 40,800

8x425 3,400

=

Average speed=11,934,000-754,800-15.81 drops per minute.

13.87

If now we calculate the total horse-power upon these dynamical averages, we have 54,400 × X 15.81 33,000-30.1 horse-power, which agrees 12 with the total from the table.

A third set of averages, which I call, for convenience, gross averages, is obtained by disregarding the number as well as the weight of stamps, and considering only the number of mills. Thus, in the case just given, the gross averages would be 637.5 pounds, 13 inches, and 22.5 drops. This has little value for accuracy; but it is the usual manner in which casual observers estimate the matter, and it shows what is the fashion or prevailing custom among owners of mills. Bearing these distinctions in mind, we have the following results, based on a comparison of thirtythree mills:

Total number of stamps, 656; average number in each mill, 19.88; total weight of stamps, 396,110 pounds; average weight, 603.83 pounds; average weight reckoned by mills, without reference to their size, 580.27

pounds; average fall in inches, reckoned from the number of stamps only, 15.34; average fall in inches, reckoned from the number of mills only, 13.41; average fall in inches, reckoned from number and weight of stamps, or average fall of the average stamp of 603.83 pounds, 13.53; average speed by stamps, 29.69 drops per minute; average speed by mills, 30.82 drops per minute; average speed of the average 603.83pound stamp, falling 13.53 inches, 28.31 drops per minute; total horsepower developed, 383; average per stamp, (obtained by dividing by the total number of stamps,) .58; horse-power developed by the average stamp at average fall and speed, (calculated from the dynamical averages,) .58, which necessarily agrees with the foregoing; average per mill, 11.60 horse-power; total number of tons crushed daily, 537; average per stamp, .82; average per mill, 16.27; total number of tons crushed by the development of thirty-three horse-powers, one in each mill, 51.16; average per mill or stamp, numerically, 1.55; actual daily product per horse-power developed by the average stamp, 1.40 tons. These figures admit of further profitable discussion.

The difference between the gross and dynamical averages of weight of stamps indicates that the larger mills carry, on the whole, heavier stamps. The difference between the gross and dynamical averages of fall is slight, while both of these are considerably less than the numerical average, showing that the larger mills, on the whole, adopt a greater fall than the gross average, but the greater aggregate weight of metal in the smaller mills nearly restores the dynamical average to the prevailing fashion, as shown by the gross average. The differences in the averages of speed are more difficult to explain. It appears that 30.82 drops per minute is the fashion, and that the few large mills running at 15 and 16 do not reduce the numerical average below 29.69. But when the fall is taken into consideration, it appears that the slow-running stamps (as might be expected) drop further, thus increasing their effect, and reducing the real effective average speed to 28.31 drops per minute. The difference between the dynamical and numerical averages of daily product per horse-power shows that the mills developing less than 11.6 horse-power crush, on the whole, slightly more in proportion than those of greater capacity; but in view of the very great variations in the final column of the table, this residual difference is comparatively insignificant, and it may be assumed that deficiencies in economy are pretty equally divided between the two classes. If the matter turned upon the daily management only, the larger mills being presumably under more skillful management, might be called upon to show better results; but the conditions here discussed are mainly those of original construction; and some of the largest mills in this table are among the oldest and the worst.

How far is this exhibit invalidated by the conditions of discharge, size of screens, etc., and hardness of rock, not included in it? By the former, I think, not to any great extent, as it may safely be assumed that these conditions have been made as favorable in every case as the form of the battery and the necessities of amalgamation will allow, and, moreover, that the mortars and screens are of one general pattern, the California high mortar not being in favor, and Russia iron, punched, being preferred to wire screens, and slits to needle-holes. Variations in the diameter of shoes are, I must confess, more common, and constitute an element which I have disregarded only because the data are wanting. But this element, if included in the discussion, would strengthen the conclusions arrived at, since the mills having the largest diameter of shoe, as the Black Hawk and Gregory, which have 9-inch