rate is lost when the discharge is not ample. The remarkable increase in product secured by the use of a double or even a continuous discharge around the whole battery-box would doubtless influence mill. men to adopt this improvement, were it not for certain difficulties, partly real, partly imaginary, in its use.

If we consider economy as well as efficiency in crushing, the advantage of a high rate is evident. With the same machinery, wages, etc., and, if the mill is well built, with little or no extra repairs, a large increase in capacity is secured. Moreover, the first cost may be reduced by the use of lighter batteries. Probably, also, the increased speed may be attained with less than the proportional increase of fuel.

The objections to higher speeds in Colorado mills are partly set forth in my last report, (pages 365-'66,) in the words of a writer in the Central City Register. His argument is, substantially, that experience has shown different rates of speed to be best for different kinds of ores. Instances are given in which, upon an increase of speed, the yield of gold per ton fell off; and it is claimed that this test should decide what rate is to be adopted in each case. In other words, the rapid running of the stamps, and consequent augmentation of product crushed, causes greater agitation within the battery-box, and requires a larger supply of water to clear the discharge and carry away the greater amount of pulp. The excess of agitation in the battery may prevent the accumu lation of gold on the interior plates, and the excess of current on the aprons may prevent the accumulation of gold there. These objections are most plausible when the gold is most finely divided in the quartz. I propose to consider them briefly.

This reasoning amounts to the confession that the conditions most favorable to economical crushing must be partly sacrificed to secure efficient amalgamation. Is this sacrifice really necessary, or is it merely involved in the method of amalgamation adopted in the Colorado mills? The attempt to catch the greater part of the gold on the interior plates interferes directly with the greatest efficiency of the stamps. The suc cess of the amalgamation at this point is in inverse proportion to the success of the crushing and discharge. There is a certain advantage gained in the force with which the pulp is dashed against the plates; but this force is liable to overdo, and thus undo, its own work, and actually remove the adhering amalgam. The same effect can be more completely secured outside of the battery.

But the arrangements outside are generally poorly adapted for the purpose. The pulp is swept over a small, steep, and smooth amalgamated surface; and it is no wonder that so little gold is caught upon the aprons. The Port Philip, Australia, mills, (see my report of 1870, p. 678) have five distinct steps or drops in the outer plates, where the Colorado mills have none. If this arrangement were adopted, ar excess of water would occasion no loss, and the efficiency of amalgamation would be increased.

The principal objection appears to be the clogging of the outside riffles or steps with pulp, or the removal of amalgam by the falling of the pulp over the steps. But it strikes me that if Australian mills can overcome these difficulties we ought to be able to do the same.

Even retaining the present patterns of outside aprons, the effect of a greater amount of water could be neutralized by spreading the discharge over a wider surface. Let us suppose, for instance, that a twenty-stamp mill is run at a low speed, for fear of losing gold if more quartz and more water were passed through it in a given time; and that ten of the stamps, run at a high speed, would have the same crushing capacity as

the whole mill at present. Why not run ten stamps in this way, and discharge upon the apron surface of the whole twenty? After the pulp is once through the screens, and sliding over the apron, it makes no difference how fast it was crushed. In a word, the conditions of amalgamation should be, and can be, regulated without interfering with the conditions of pulverization. Loss of gold should be, and can be, prevented without crippling the efficiency of the stamps. Power, space, and time are at our disposal; and by a proper use of the two latter we may avoid wasting the first, which is the most costly.

My views on this subject may be summed up as follows:

1. The stamp-mill is the most convenient and practically efficient machine for crushing quartz thus far introduced and proved by experience. It involves little waste of power in gearing; it delivers its power in the most direct and practical manner, namely, by blows, which take advantage of the brittleness of the rock, instead of pressure or friction, which invite the resistance of hardness; its capacities for charging and discharging are ample and easily regulated, both as to quantity and as to fineness of the product; it is subject to few and comparatively inexpensive repairs, and it can be repaired, in most cases, without complete stoppage. These and other excellent features in its construction and operation render it especially suitable for use in mining districts remote from machine-shops, founderies, and centers of skilled labor.

2. To obtain the best results, stamp-batteries should be built and run to secure the highest efficiency and economy in crushing only, without reference to amalgamation. The amalgamating apparatus should be adapted to the batteries, not the latter to the former. If interior plates are employed, they should not be expected to catch the greater part of the gold, nor should the pulp escaping through the screens be swiftly and carelessly manipulated, when a little extra space and time devoted to it, almost without extra labor, would avoid much loss.

3. The efficiency of a stamp may be described as the product of three factors-weight, fall, and speed. The efficiency of a battery of stamps involves a coefficient-the discharge.

4. When the fineness of crushing is regulated by screens, the discharge should be as large as practicable. There may be mechanical ob-" jections to continuous screens running around the whole battery; but there are, I think, no valid arguments against the double discharge, in front and rear, when the battery is properly planned with reference to it. Of course a feature of this kind cannot always be successfully added, like a patch, to a battery not duly proportioned for it.

5. Of the three factors of the efficiency of the stamp, the weight and fall determine the force of the blows, and the speed determines their frequency. The height of fall is practically limited by the speed, and by considerations of mechanical convenience.

6. Within certain limits, light blows, frequently repeated, are more efficient than heavy blows at longer intervals. These limits are the following: The stamp must be heavy enough to work steadily, and fall far enough to allow proper feeding and distribution of the ore, and to produce the splash necessary for effective discharge. (In many cases, by the way, more weight might be advantageously put in the stems, and less in the heads.) Again, the blow must be heavy enough to crush the rock upon which it falls. If too heavy, it may waste power in packing the crushed rock; if too light, it may fail to crush, and so may pack. Finally, the speed should not be so great as to prevent proper clearance, or the stamp may strike a second blow upon the rock already crushed. 7. The efficiency of a blow from a heavy stamp with short drop is less

than that of an equal blow (in foot-pounds) given by a lighter stamp with longer drop-the practical limits already referred to being observedbecause the longer drop gives greater final velocity to the stamp, and this tends to crush more and to pack less. The same principle underlies the effect of nitro-glycerine, as observed at the bottom of blastingholes, where the rock in the immediate neighborhood is shattered and pulverized by the suddenness of the explosive shock.

8. The superior effectiveness of frequent blows lies in the fact that there is a limit to the amount of crushing which can be practically performed by a single impact upon a given quantity of rock distributed over a given surface. Thus, a thousand foot-pounds, delivered instantaneously upon a surface eight inches in diameter, may be resolved into six hundred of minute motion or crushing, and four hundred of gross motion, or packing, and heat; while five hundred foot-pounds, under the same circumstances, may perform four hundred of crushing, and waste only one hundred. Two of the latter blows would then effect more with the same force than one of the former. There is another practical advantage of high speed. If stamps are left, as it were, standing in the pulp, between blows, the material settles around them and they "suck" when the lift commences. A great deal of power is frequently wasted in this way, by not picking up the stamps before they become partially buried.

9. But even if the efficiency of stamps were always exactly measured by the product of the three factors mentioned, that is, by the number of foot-pounds delivered per minute, (which is certainly not the case,) there would still be good reason for preferring rapid running. After the necessary stability and strength are secured, increased weight of machinery is an evil. If equal results can be achieved by substituting speed for weight, the change is advisable.

10. In the case of the Colorado mills, the argument is still stronger. Their (gross) average weight of stamp, 580 pounds, is not excessive; their average drop, 134 inches, is not too large to admit of high speed; but their average speed, say 30 drops per minute, is extremely low, and might be doubled with advantage. A bad arrangement for amalgamation is one excuse, which should be removed, not pleaded. Another serious objection, which Colorado experts are not so free in expressing, is a bad construction of battery foundations and frames. It is feared ' that high rates of speed would rack or upset the batteries. The difference in this respect between the mills of Colorado and those of other regions may be seen by comparing the drawings given in a previous chapter of this report with that on page 664 of my former report. The California mortar rests on a vertical block, and the blow of the stamp does not communicate vibrations to horizontal timbers.

I believe the views I have expressed are coming more and more to be those of American millmen, even in Colorado. The true evidence of this tendency is to be found in the patterns of the new mills, rather than the practice of those persons who are frequently obliged to adapt themselves to the proportions or condition of antiquated machinery. Moreover, the manufacturers frequently adhere to the old patterns, or at least put higher prices upon machinery constructed after new ones; and few engineers have the opportunity of dictating from their own experience the details of their mills. Mine-owners think a stamp is a stamp, and a steam-engine a steam-engine; and desiring so many stamps with so much horse-power to run them, pick up what they want wherever they can get it most cheaply-at second-hand, if possible. But many causes, and particularly the keen competition among custom-mills, are bringing about a wholesome progress in this matter.

CHAPTER XIII.

THE WASHOE PAN AMALGAMATION.

The third volume of the Report of the United States Geological Exploration of the Fortieth Parallel contains an admirable chapter, from the pen of Professor J. D. Hague, on the treatment of the Comstock ores. As the expensive character of that work, and the comparatively limited edition of it published by the Government, prevent its general circulation among the classes most interested in this part of its contents, a portion of the chapter referred to will be here abridged, with such notes and comments as may seem useful.

The division of the Comstock ores into first, second, and third class is arbitrary and variable, having reference rather to the treatment chosen for each class than to the mineralogical constitution of the ore. The first class receives the most careful treatment, and usually possesses an assay value exceeding $150, or even $100, per ton. The second class, where it is distinguished at all, usually includes ores assaying from $90 to $150. The third class comprises all workable ore of still lower grades. The first-class ores form but a small proportion of the whole. For instance, the Savage mine produced, in the year ending July 1, 1868, 87,341 tons of ore, yielding an average of $40 84 per ton, of which only 277 tous were first class, having an average assay value of $449 40 per ton, and an average yield of $359 52; and 4,745 tons were second class, with an average assay value of $124 25 to $142 82, and yielding $78 16 per ton. The remaining 78,432 tons of third-class ore assayed $52 01 to $55 11, and yielded an average of $37 20. In the following year, out of a total of 69,287 tons, there were only 681 tons called first class, and having an average assay value of $275 47, while there was no second class distinguished, and 55,411 tons of the third class, assaying $50 78 to $60 29, yielded $34 64 per ton.*

About 25 to 30 per cent. of the value of these ores is gold, and the remainder silver. In the bullion produced the relative proportion of the gold is a little higher, as it is more completely saved than the silver.

The first-class ores are treated with dry crushing, roasting with salt, and subsequent amalgamation. The ores of the second and third classes are subjected to the "Washoe" process proper, as follows:

Crushing. This is universally performed in stamp-mills, the larger pieces being "spalled" to a suitable size for feeding into the batteries. For this purpose Blake's rock-breaker is frequently used instead of the hand-sledge.

The foundation of the battery is like that adopted in California, consisting of heavy vertical timbers, firmly bolted together, and tightly packed with clay or earth. The mortars are usually placed directly upon these vertical mortar-blocks. The mortar in general use for wet

* The earlier operations of the Comstock furnished a much larger proportion of rich ores, partly because the rich ores were eagerly extracted, and those of lower grade left standing. The greater part of the product of late years has been from material overlooked or discarded by the extravagant managers of the "flush times" of Washoe. It would be unfair to argue from the figures that the vein has to this extent "grown poorer;" they rather show that the operations of extraction and reduction have become cheaper, more skillful, and more rational.-R. W. R.

+ Generally assumed, roughly, at one-third the value.-R. W. R.

Differing from the Colorado plan, as will be seen by reference to the chapter on that subject in this report.-R. W. R.

crushing is an iron box or trough, 4 or 5 feet in length and depth, and 12 inches in inside width, cast solid. The feed-slit is 3 or 4 inches wide, and the discharge-opening is 12 to 18 inches high, the lower edge being 2 or 3 inches above the top of the die. The single discharge is generally used. The screens are of brass wire-cloth, 40 to 60 meshes to the inch, or (as is preferred for wet-crushing) of Russia sheet iron, perforated with holes to inch in diameter. The dies are cylindrical, 4 to 6 inches high, and usually cast on a square flat base, with truncated corners, so as to fill the bottom of the mortar, and yet be easily removed when necessary.

The stamp stems are usually of turned wrought iron, about 3 inches in diameter, 10 to 12 feet long, and slightly tapered below to fit the sockets in the heads. The latter are cylinders of tough cast iron, about 8 inches in diameter and 15 inches high. The socket for the stem is about 7 inches deep. A similar, but larger, socket in the lower end of the head receives the shank of the shoe. Each end of the stamphead is encircled with a stout wrought-iron hoop, shrunk upon it like a tire.

The shoes are usually about 8 inches in diameter and 6 inches high, with a tapering shank about 5 inches high and 4 to 5 inches thick where it joins the shoe proper. They are made of the hardest white iron,* and are replaced when worn down to about one inch in height.

The collar or tappet, preferred in California and Nevada, is Wheeler's gib-tappet, which is cylindrical in form, (effecting the revolution of the stem during the lift,) and differs from others of that pattern in the manner of its attachment to the stem. This is effected, not by tapering the stem or cutting the screw-thread or key-seat upon it, but by means of a gib and two keys, which clamp the collar to the stem at any desired point.

The

The rotary motion of the stamp, imparted by the friction of the cam against the tappet, is in very general use in Nevada. This is one of the advantages offered by the use of round shoes, stems, and tappets. revolving cam, meeting the tappet and raising the stamp, causes it, while being lifted, to make a partial revolution about its vertical axis, which rotary motion being continued during the free fall of the stamp, produces a grinding effect between the shoe and die upon the substance to be crushed. Not only is the effective duty of the stamp at each blow increased in this way, but the shoe wears down much more evenly than when it falls without such rotary motion.†

The guides, which are of wood, and supported by the cross-timbers of the battery-frame, are placed, one set below the tappet, about a foot above the top of the mortar, and the other set near the top of the stem, so that six inches or a foot of the latter may project above.

*The manner in which shoes, heads, and stems are attached together in practice is described in the chapter on the Colorado process in this report.-R. W. R.

I have copied this paragraph verbatim from Professor Hague's chapter; but I must take leave to doubt the existence of an effective grinding action, such as he describes, at least from stamps run at ordinary speed, say 30 to 70 drops per minute. The circular revolving stamps have their advantages, no doubt; the chief ones being convenience and regularity of wear. But their dynamic advantage, if it exists at all, is much overrated, as the statistics of the best square stamps will show. If I remember correctly, some comparative tests, made under the superintendence of Mr. S. S. Robinson, in one of the largest stamp-mills of the Lake Superior copper region, did not indicate a greater crushing capacity for the revolving stamps. And it may well be questioned whether the most recent German batteries (which still retain the square stamp) are not as effective as our own.-R. W. R.