16 BULLETIN 77, U. S. DEPARTMENT OF AGRICULTURE. 



treatments with creosote, carbolineum, or crude oil. One operator 

 referred to the disagreeable effect on the miners of the odor from the 

 creosote in the mine. 



There was great variation in the replies to the question regarding 

 the proportion of timber used for replacements made necessary by 

 decay. Replies from 30 coal mines averaged 11 per cent (14 reporting 

 per cent and none 100 per cent), and from 112 metal mines aver- 

 aged 21 per cent (44 reporting per cent and 11 reporting 100 per 

 cent). It is apparent that the economy of preservative treatment is 

 a local problem with each mine. 



In order to decide whether it would be economical in any given case 

 to apply treatment only a few factors concerning the present costs 

 and life of the timbers are essential. The proper method 1 of treat- 

 ment would depend upon species of wood, capacity of apparatus, and 

 local conditions, but the preliminary steps could be taken on the basis 

 of the following data : 



1. The amount and cost of material that goes annually toward 

 replacing decayed timbers. 



2. The cost, in place, of the timbers used. 



3. The average life of the timber. 



The first of these factors will readily show whether the amount 

 concerned is large enough to warrant further consideration of pre- 

 servative treatment. In certain coal mines, where but few timbers 

 are necessary in the permanent entries and where the majority of 

 timbers are used in temporary positions, a brush treatment would 

 probably be all that is warranted, unless impregnated material could 

 be procured in the market. On the other hand, where a large amount 

 of material is used to replace decayed timbers the installation of a 

 small treating plant might be seriously considered. 



From the cost in place and the average fife of the timbers there can 

 be calculated an annual charge, a measure of the relative economy, 

 with which any other combination of cost and life (such as that 

 resulting from a preservative treatment of the timber or perhaps 

 from the use of some other kind of material) can be compared. In 

 figure 5 is presented a series of curves, each for a definite initial cost 

 of a timber or set of timbers, which show the relation between length 

 of life and annual charge when compound interest on the investment 

 is considered and maintenance is assumed for an indefinite time; 2 



> Forest Service Bulletin 10", The Preservation of Mine Timbers, by E. W. Peters, treats of various 

 methods of preserving mine timbers from decay. 



2 The annual charges are figured on this basis so that a fair comparison can be made between different 

 methods involving various combinations of costs and length of service. If a material were placed that would 

 last forever its annual charge would be simply the interest on the cost in place. If the material must be 

 replaced every so often, the annual charge represents the interest on a sum of money which, if placed at 

 compound interest, would pay for the initial cost in place and all future replacements and would be itself 

 used up at the end of the period of maintenance under consideration. If this period be of an indefinite 

 length (forever), the annual charge is the interest on a sum of money sufficient to pay the present cost of 

 installation and, by the accumulation of compound interest on the balance, take care of future replace- 

 ments at the assumed intervals of time. 



