Having given the above condensed descriptive notice, we shall conclude by talking a brief review of the following questions con­nected with this engine. The conditions of working, and the results obtained, and ob­tainable. The types and power of motive-engines most applicable. Considerations of consumption, expense, and general advantages of man-engines, and particularly of the single-rod type. And a review of the comparative advantages of the man-engine, and other apparatus for raising miners. Condition of working and results. In most metallic mining districts the day of twenty-four hours is divided into three cores (or corps) of eight hours each, thus distributed: from 6 a.m. to 2 p.m., from 2 p.m. to 10 p.m., and from 10 p.m. to 6 a.m., the last core being often wanting. Where a shaft is sinking there are frequently four cores of six hours each, the sump men changing at 8 and at 2.To meet these various, requirements the engine at Dolcoath is worked thirteen hours out of-twenty-four, as follows: from 6 to 9 a.m., from 2 to 8 p.m., and from 10 p.m. to 2 a.m. At Levant they work seven hours out of the twenty-four: from 6 to 7a.m., from 2 to 6 p.m., and from 8 to 10 p.m. It follows from this, that man-engines are far from being com­pletely utilised, that is, the rods are never constantly full. In order to arrive at the maximum power of these machines, it will be neces­sary, for a moment to imagine a working by which they shall be fully employed. As in both systems, the ascending and descending currents go on simultaneously, it will be only necessary to consider one of them. In the single-rod type every platform may be manned in each up or down stroke, and in the return stroke they can also be manned by those moving in the opposite direction; no platform, in any stroke, need ever be empty. In the double-rod type, on the contrary, the miner only returns to the same rod at every other platform, leaving the intermediate ones as a distinct route for the opposite current; so that, on any one rod, only half the platforms can, under any circumstances, be occupied at the same time. Now, if L be the length of the man-engine in fathoms, and the platforms be always two fathoms apart, L÷2 = P will be the number of platforms on each rod. Let s be the number of strokes per minute made by each rod in the double-rod system, and s the number of strokes per minute made by the single rod. Let n be the number of miners to descend; and let t and t1 be the corresponding times, in minutes, required to send them down by the double and single-rod systems respectively. In the double-rod system, the leading miner of the descending current will arrive at the bottom of the shaft after having occupied successively P ÷ 2 platforms on one of the rods, which will require in time ½ P ÷ s  minutes.   After the arrival of this leading man of the current, each stroke of the same rod will bring another, so that n miners will be brought down in — minutes; so that we have:- t=½ P + n ÷ s……………(a) With the single rod type of engine, where the miner must occupy successively all the platforms of the rod, we have similarly:-

t’=½ P  +n ÷  s’……………(a)

Applying these formulae to a given case, so as to compare the efficiency of the two types of engines, let us take that of a man-engine extending to 220 fathoms deep, having to send down a core of 200 men: here L = 220, P = 110, and n = 200.

In the double-rod type, working at the rate of the United Mines engine, three strokes per minute, where, consequently, s = 3, we have, by formula (a) t’= 55 + 200 ÷ 3= 85 minutes, the time required to send down the 200 men by this form of engine, In the single-rod type, working at the rate of Carn Brea engine, four strokes per minute, and where, consequently, s’ = 4, we have, by formula (b) t ’= 100 + 200 ÷ 4 = 77½ minutes, the time required to send down the 200 men by this type of engine. A result which shows a balance of  7½ minutes in favour of the single-rod arrangement, only working one-quarter quicker. Indeed, a simple consideration of the subject ought to show the great superiority of the single-rod arrangement. For even if the two types of engine worked at the same rate, although the double rod would send down any one man in half the time required by the single rod, the time occupied in sending down any given number of men would be the same m both cases when the current had been once established. But the double-rod engine cannot be worked at the same rate as the single one; for experience shows, that the number of strokes per minute of the former cannot safely exceed half the number of strokes which may be given to the latter: that is, if the one goes 3 strokes per minute, the other may be worked to 6. Accepting, therefore, that while s = 8, s’ may equal 6, we see that the single rod will send down any single man as quick as the double rods, and any given number of men in half the time, the current once established. (It is rather remarkable that there seems to be a wide-spread misunderstanding in Cornwall as to the respective capabilities of the two types of man-engines. While the single-rod type has of late been exclusively adopted, there seems yet to be a notion that the double rods are able to do twice the quantity of work, and are only not adopted because such an amount of work is not required.) To get a general formula for ascertaining the number of strokes per minute, s’ required to be given by a single-rod engine in order to perform the same amount of work as two rods each making s strokes, where, consequently, t = t’, we have from formula (a) and (b):-  ½ P + n ÷ s’ = P + n ÷ s’ or s’ = 2s P + n ÷ P + 2n ………..(c)

    From this equation it follows:-

    1. That s’ is always less than 2 s.

    2. And smaller, in greater proportion for the same depth, as the number is greater;

    3. That for any given number s’ will increase with the depth. To make these conclusions more intelligible, let us take the case already given where 200 men were lowered to a depth of 220 fathoms in 85 minutes by a double-rod engine working 3 strokes per minute; and let us ascertain by equation (c) the number of strokes per minute of a single-rod engine would be required .to do the same work. Here s’ = 2 x 3 -  110 + 200 ÷ 110 + 2 x 200 = 3.64, that is to say, a little more than 3½  strokes per minute of the single rod would suffice to do the work in the same time as performed by the two rods working each at the rate of 3 strokes per minute. Assuming that s’ = 2 s; and taking the case of a deep mine (say 290 fathoms) sending down 500 men per day, 200 in each of the day cores, and 100 in the night core, we shall find that while the total time required for sending down the 3 cores with the double-rod engine (working 3 strokes per minute) is 4 hours 32½ minutes, the time required to do the same work with the single-rod engine (work­ing 6 strokes per minute) will only be 2 hours 52½ minutes, showing a saving of 1 hour 40 minutes, or more than one-third of the whole time. Where the motive-engine is employed for other purposes, such as drawing stuff, this saving may be of much importance. Motive-engines:- In calculating the useful power required to work a man-engine, it is necessary to consider that besides the useful work performed, there is a dead weight to be overcome in the fric­tion and the slight unbalanced weight of the rods. This dead weight of course varies considerably with local circumstances, and particularly with the underlie of the shaft; at the United mines it was estimated that one-third of the motive power was absorbed in the friction resistances. The weight of a miner may be taken at 150Ibs., and as it is necessary in calculating the power required to assume the maximum that the machine could hold, we shall have, taking account only of the ascending current. In the double-rod system, on one of the rods (take account only of the ascending current) P ÷ 2 platforms are occupied. During one minute these will receive 2 x s strokes of 12 feet, the useful power (x) of which, expressed in pounds raised one foot high, will be:-

x = P X 2.s X 12 X150; or x = P s. 12.150   ....    (d.)

In the single-rod system, where the whole number of P platforms are occupied during the stroke of 12 feet, we have similarly:-

x’ = P . s’. 12.150 .......    (e.)

Thus expressed, x and x’ have the same form, which we can readily understand, for the P ÷ 2 miners of the two rods are constantly in movement, while the P miners of the single rod are half their time on the fixed sollars. If we apply these formulae to the cases of the United mines and Dolcoath, dividing the results by 33,000 to get the horse-power required, we have:- United mines  x =105.3. 12.150 = 567,000 = 17 1/8 horse-power.

    Dolcoath x’ = 110. 5½. 12. 150 = 693,000 = 21 horse-power.

From this hypothesis, which assumes only one ascending current, let us turn to the opposite one, and consider the two contrary currents in motion ; we here find that in the double-rod type there is a permanent equilibrium, the same number of miners always occupying the two rods, but inversely changing from one to the other. In the single-rod engine, on the contrary, the equilibrium is alternative, so to say; that is, if the machine is provided with a suffi­ciently powerful fly-wheel to store up the motive power derived from the weight of the descending miners, this power will be given out again in the following ascending stroke. The excess of the weight of the rod unbalanced produces the same effect, so that in the two stems there is nothing to overcome but the friction. These observations clearly show the advantages of the single-rod type of engine. If we suppose the shaft vertical , and s = 2s, we shall find that the friction in the guides, &c., without being entirely independent of this relation, is far from being proportional to it. The single rod, with its double weight, and with a movement equal to that of the two rods, will not have double the friction, while during its work it will bring up and send down twice the number of men. Practically, man-engines usually work between these extreme limits; all the platforms are not manned, and the contrary currents are not always equal. However this may be, by the adoption of the single-rod machine, we can profit by the motive -power given out by the weight of the descending core by using a rotary engine with a powerful fly-wheel. As to the type of motive-engine most applicable to the man-engine, it would appear at first sight that, as a reciprocal motion is re­quired, the ordinary form of pumping engine would be easiest applicable. However, sound practical considerations have proved that for both systems, the double and the single, rotary engines are the most suitable. The objection which a priori would suggest itself to the rotary engine is that at the end of the stroke corresponding to the dead point there is, properly speaking, only a slackening of speed, not an absolute stoppage. No practical inconvenience, however, results from this in consequence of a certain play of "the rods due to their elasticity and mass, and to the great slowness of the movement near the dead point. It is generally held that, with a stroke of 2 fathoms we cannot safely exceed a rate of 3 strokes per minute in the case of the double-rod type of engine, and 6 strokes per minute in the case of the single-rod type. These rates are evidently equivalent as to the time left to the miner, when we consider that in the one case both plat­forms are movable, while in the other case one is movable and the other fixed. A simple trigonometrical calculation of the arcs tra­versed by the crank at the various periods of the stroke, and of the time occupied, will give us the space traversed by the rod within such periods, and establish this point. M. Moissenet, in his memoir in the Annales des Mines already referred to, has also shown that, by an arrangement which he suggests, the single-rod type of engine could safely be worked as fast as even 8 strokes per minute. In this arrangement the comparative rate of the movement of the rod is expedited in the middle of the stroke, but retarded towards the beginning and end; so that, although the total time occupied by each stroke is reduced from 10 seconds to 7½ seconds, the rate at the beginning and ending of the stroke is not increased. With regard to safety, there can be no doubt that direct-acting engines, leaving an absolute interval of repose at the end of each stroke, would generally be less safe than the rotary engines now used, which only give a very slow movement about the dead point. The man-engine rod, like pump rods, would then have to start suddenly into motion on the admission of the steam into the cylinder, which would evidently give rise to much more danger than the present mode of slow acceleration at the commencement of the stroke. Another important economical consideration also leads us to decide in favour of the rotary engine. As the man-engine is not required to work continuously, but yet at such frequent intervals that the steam must be always kept up and the engineers on the spot, it is import­ant that the engine employed be of such a type that it may readily be applied to other purposes. Now the rotary engine is the only one so applicable, as the direct acting engine can never be used for any other work than pumping, and indeed, as will be shown further on, it would not even be good for much for this purpose if it were modified so as to be safe for the man-engine. Besides, a steam-engine of the power required to work a man-engine in a deep mine, about 35 horse-power, is just the engine required for drawing from a similar mine. Such a sized engine would be useless for pumping in a deep mine, and, in addition, in an immense majority of cases the pumping is required to be continuous and not intermittent. In one case in Cornwall a direct-acting engine has been adapted to work the Wheal Reeth man-engine, by Mr. George Eustice, jun., of Hayle. In this case the arrangement was adopted in order to utilise an old 30-inch pumping engine which happened to be on the mine, and not deliberately adopted as the most advisable course in case entirely new machinery were being erected. As similar condi­tions may occur in other mines, in whose special case it may be advisable to adopt a single engine working the rod directly, it may be well to state the modifications which Mr. Eustice has used in r order safely to apply this form of engine to working a man-engine. In the first place no expansion of steam is allowed, for this would necessarily be productive of great danger from the sudden shock given to the rod at the commencement of the stroke; the valves are open throughout the whole length of the stroke of the piston. The amount of steam admitted into, and consequently its pressure in, the cylinder is regulated by the engine-man according to the number of men on the rods; if too much steam is admitted the engine will come “in-doors” too fast, and if too little it will come in too slow. In the case of the rod being heavily laden by men going down, it is also requisite to take precautions against its going “out-of-doors” too fast, which would necessarily occur if, at the end of the in-door stroke, the equilibrium valve were suddenly opened with a rod heavily manned. For this purpose a throttle-valve is placed in the top of the vertical pipe connecting the equilibrium valve; by this the rate at which the steam is allowed to pass from the top to the bottom of the cylinder is regulated at will, and with it the rate at which the piston ascends. The whole of this arrangement is very creditable to Mr. Eustice, for circumstances may occur where the adoption of an engine of this kind would be economical, and before he took it in hand it was deemed impossible safely to apply a single-acting engine for the purpose. Consumption and Expenses of Man-Engines:- It must be evident that the consumption of coals, grease, &c., and the other steam-engine charges, must vary with the varying circumstances; and besides, as the motive-engines are usually employed in doing other work, it is not always easy to apportion the respective proportion of cost which should be set down to each. At the United mines the cost on the man-engine has been estimated at £30 per month. In the case of Levant, where the motive-engine also draws from a depth of 79 fathoms, and where its total cost is £25. 5s per month, M. Moissenet calculates that 3/8^ths of this should be apportioned to the man-engine, which would give £15. 8s per month. If to this we add 17s for grease, &c., used underground, we have a total monthly cost of £16. In the case of Dolcoath, similarly, where M. Moissenet apportions half the steam-engine cost to the man-engine, he gets a monthly charge of £16. 8s, which, however, is probably much under the mark. As we shall take the opportunity of giving on an early occasion detailed particulars of the monthly cost incurred in working the various man-engines in the county, together with the number of men transported, and the depths, it is unnecessary to dwell further upon these generalities. Advantages of Maw-Engines, particularly the Single-Rod Type:- The enormous loss of labour and time incurred in climbing ladders is well known, indeed, too evident to make it necessary to dwell upon it. To go down by ladders to the depth of 250 fathoms a miner will occupy about 40 minutes, and to climb the same distance he will take about 1 hour, or 1 hour 40 minutes in the descent and ascent. In the man-engine he can go up and down in 25 minutes each way, 50 minutes in all, which gives a saving of 50 minutes, or more than 1/16^th, in time of the miner’s working day. The saving in fatigue and labour probably amounts to as much more, so that the saving by the application of mechanical means varies from 1/5^th to 1/6^th, and is certainly never less than 1/6^th, on the labour expended. , This is the mere money question, but besides this there is the question of health. What has been already said must have sufficiently demonstrated the superiority of the single-rod type of engine; but still it may be well to give a summary of its advantages in a short compass.

1. It is less expensive, and occupies less space. 2. It is safer in several respects.   In the first place, the shaft at each two fathoms may be sollared over, only leaving the man-hole, so that in case of a man falling away there would be a fair chance of escape. In the next place, the danger of stepping on to a moving platform is much greater than on to a fixed one; and besides, in case of a man getting giddy, he can rest himself as long as he desires on any sollar, while in a similar case in the double-rod engine he would be carried up and down with each stroke of the engine, which would very likely make him worse and insure his destruction.

3. As it can be worked twice as quick, each man can be sent down in the same time as with the double-rod engine, and the same number of men can be sent down in one-half the time, when the current is once established, a matter, of course, of immense importance

s = L÷ d.s + n ÷ s

Putting d s  = V the velocity of transportation, we have:

t = L ÷ V + n ÷ s

which shows the time necessary to send down n men to a depth of L fathoms, making s strokes per minute of d fathoms each. The activity will be greatest when the values of d and s are such as to reduce t to a minimum. Comparative advantages of man-engines and other mechanical means for raising miners:- This very important question, which, as has been said, has recently been much discussed in Cornwall, is one on which a good deal may be said on both sides. In favour of the man-engine we have the following considerations:

1. It is safer, that there can be no doubt about.

2. Being once fixed in place, it will send down or take up a greater number of men in less time, and at a less cost, than can be accomplished by any other means; this is equally clear; for by no other appliance can we deliver a continuous stream of men to a depth of say 300 fathoms, at the rate of 360 men per hour, with engines of the size described.

3. Any man can leave or get into the engine at the level of any sollar, which in metallic mines is a matter of great importance, where workings are carried on at so many different levels. Against the use of the man-engine as compared with skips or other similar appliances used in collieries, it has been urged:

1. That although the man-engine may be the safer, yet as it can be so seldom applied (seeing that there are only 8 in Cornwall), its
general non-applicability, which still perpetuates so much climbing, really causes more injury to health and life than would be at all likely to arise from the general practice of sending down men in well-arranged skips with wire ropes.

2. That although cheaper and more expeditious when once esta­blished, yet, since it necessitates the putting up of special machinery at an extra cost, and above all requires an extra space (not often available), it really brings little economy in the end.    On this  point, the advocates of the skip also urge that the general application of the same means to raising men as are used for raising ores, would necessarily lead to an improvement of the latter, in which Cornwall is undoubtedly backward, and consequently lead to great economy. It is quite clear, regarding the matter candidly, that there are points to be urged on both sides. Skips with wire ropes, such as men might be sent down by with reasonable safety, ought to be on every mine, and in every working shaft, for the sake of economical drawing alone: man-engines can never be so commonly used. Hence, the rational views would seem to be these:-

1. Where the depth of the mine, the number of men employed, and the general prospects are such as to justify it, and where the
extra space required is available without much increased cost, the putting in of a man-engine is the most advisable course.