siderable recovery of coal that could not have been mined had the workings not been silted.

The first example under the section on "Silting" brings to notice the results that may follow the more extensive adoption of the practice. The second example in this same section shows another method that one of the coal companies has devised. As it happens, both methods have been developed by the same company, the one which has studied the matter most carefully.

To deposit on the surface the large rock and refuse banks that are now being put there throughout the region seems largely unnecessary. A great amount of this material could be put back into the mine where it belongs, although under some conditions the procedure would not be advisable. Certain mines have been worked to such a point that they can not take care of the material that is produced, but ordinarily just the reverse is true, and there is not enough material to silt the mines properly.

Silting has been used extensively in the built-up areas of the anthracite region and under the buried valley to give additional support to the surface, but no matter how carefully an area in the mine is silted there is naturally some settling of the surface if subsidence occurs, for the silt can not be packed in as tightly as the original coal and is subject to compression.

A survey of 106 collieries in the anthracite region indicates that only 29 are silting, while the other 77 deposit most of their refuse on the surface and do not use it to prevent caving and subsidence; that is, silting methods are being used at 27.4 per cent of the above collieries.

MISCELLANEOUS MINING METHODS

In the section on "Miscellaneous mining methods" various types of mining have been described and illustrated by examples. Some of these methods are so out of date that they may be ignored; in fact, it would benefit the mining industry if they were abandoned entirely, particularly the block system, buggy-road mining, and room-and-pillar mining in thin beds, for much better sj-stems could replace any of them to advantage. However, there may be a few mines where the remaining territory is so small or the working place is so isolated that a change now would be inadvisable.

The account of the attempt to reopen the Hillman bed is of interest to operators in the anthracite region, for there are extensive areas that have been first mined and where present conditions are similar to those described. Probably thousands of acres have been first mined and allowed to stand many years. Now fallen rock has filled sible. New gangways have to be driven to reopen territory of this character. Much better results could, no doubt, be attained if some mechanical means were provided for removing the fallen material in the rooms as the gangways and airways cross them. However, this point will be discussed at some length in the next section, under "Conclusions."

The description of mining under the Susquehanna River at the Seneca colliery is of interest as shewing the possibility of recovering large areas of coal that lie under the river and under the buried valley. As the depth of the workings under the river and the valley increases, other methods of mining will possibly be developed that will insure a higher percentage of recovery. The section giving conclusions will present some pertinent suggestions.

The methods given under " Conservation of small areas of anthracite" show how far a company is willing to go to conserve its coal supply. However, not only is the company conserving its coal but it is actually saving money, for if it left these areas behind without mining them it would have to go back after them some time later, and the increased expense then would make the cost of production much higher than the cost of mining at present.

MECHANICAL MINING

Except for the incidental statements made in the various descriptions of mining methods, nothing has been said about the mechanical mining of anthracite. Many of the descriptions have mentioned undercutting of the coal, and some indication has been given of the economies of this method of mining. In the anthracite region there are a number of undercutters (mining machines), as the accompanying table shows.

Table 42.—Mining machines

This table shows that the number of undercutters remained about constant from 1916 through 1920, but the number had dropped in 1924 and the production of machine-mined coal had fallen to about two-thirds that of 1916. The output for 1920 was low, as a strike was in progress. The figures given in the table for the tons of coal produced are not entirely accurate, as in 1920 some companies did not report the amount, having discontinued keeping a record of it.

So far as has been ascertainable no belt conveyors are used underground in the anthracite region. No coal-loading machines are being operated, but seven rock-loading machines are giving excellent results, and the indications are that more of them will be put into use within the next few years, as soon as their value is fully realized. The statement that there are only seven rock-loading machines in use may not be entirely accurate, as no information on this subject was obtainable from two of the largest companies, which quite possibly may have some.

The total number of mechanical mining methods in the anthracite region for 1924 by inspection districts, with reports lacking from the first and the eighth and part of the second, is shown in the following table:

Table 43.—Number and kinds of mechanical mining methods and machines in use in anthracite region1

MINING OF THIN COAL BEDS IN PENNSYLVANIA 103

After giving much thought and study to the mining of thin beds in the anthracite region of Pennsylvania and visiting and examining the installations that have been described herein, as well as others so similar that their description would be of no value, the author believes there are a number of points that would well repay careful study.

INTRODUCTION OF NEW MINING METHODS

Possibly the most important feature in starting any new type of mining is the need of interesting the operators and engineers in the advantages to be gained. As knowledge regarding the mechanical handling of coal at the working face and from the working face to the main haulage roads is relatively superficial, it will be necessary first for some engineers and operators to study the methods now in use in the bituminous fields and to determine how far they may be adapted to the anthracite region. Without accurate data on operation, capacity, cost of upkeep, and reliability, it is difficult to reach any conclusions as to the fitness of the machines for use in the anthracite region.

The second step is the arousing of interest among the men who will have immediate charge, for without close cooperation from the bosses a new type of mining, no matter how excellent, must be stamped a failure from the start.

The third step is to interest the Avorkers who use the system. "Unless they adopt it whole-heartedly they will surely cause it to fail, partly or wholly; unless there is proper interest, nothing is or can be a success. Workers must be tactfully convinced of the advantages of a mining machine before they will heartily approve its use. The results obtained under the old way of introducing a new method are typified by a colliery that installed a certain piece of mining machinery and told certain men to use it. They were given no choice and were to be paid a rate set by the coal company. The advantages were not explained, the men were not shown how this piece of machinery would be of benefit to them, and they said, "We will not operate it or use it." It has never been operated. However, it could have been operated with success had the men been shown that the company was not getting all of the benefits but was sharing them with the men.

On page 10 it was shown that the average percentage of recovery for 10 places in the anthracite region was 82.4 per cent, which was indicates to some extent what might be expected if a more concerted study of thin-bed mining were made and modern mechanical appliances used for mining and for transporting coal from the working face "to the gangways. Although these 10 places show this increased recovery, their average probably could be increased by some changes, for the work being done is largely experimental.

ADVANTAGES OF LONGWALL AND SEMILONGWALL MINING

Longwall and semilongwall methods of mining have not been used with success in this country. Of course, there are some outstanding exceptions to this statement, but, taking the country as a whole, it is correct for both bituminous and anthracite mines. Numerous reasons have been advanced from time to time as to why longwall mining has not been employed more, but they do not seem justifiable.

The author believes that the chief reason that they have not been successful is that the American operator or mine owner is not willing to go to the expense of back filling. If longwall mining could be done without back filling, he would be willing to try it. He is willing to try it if he can use timber, but timber often does not support the roof adequately; when breaks occur they are liable to carry to the face, and the face is lost.

The back filling of the longwall place with rock gives immediate support to the roof, and settling takes place gradually, not with a rush. The more the roof settles the greater the resistance given by the back fill and therefore the more support. With other methods of support the greatest resistance to settling is at first, and as the load increases the supports fail, resistance is lessened, and the roof breaks badly.

SAVING POSSIBLE

What saving would be possible in the anthracite region by introducing longwall methods more widely? Of the 20,770,916,000 tons estimated as contained in all beds when mining began, 3,455,910,000 tons, or 16.6 per cent, were in thin beds, that is, beds 5 feet or less in thickness. On the assumption that 16.6 per cent of the now remaining coal, 16,354,675,000 tons, is in thin beds, the total thin-bed coal would amount to 2,720,000,000 tons. According to the present rate of recovery about 800,000,000 tons of coal—10 years' supply for the country—will be lost in mining, besides that lost in preparation and