152 THE BOOK OF FORESTRY 
depth of an inch to one and a half inches, it is con- 
sidered sufficient. 
Piles and paving blocks are being treated extensively 
at present but the preservation of railroad ties consti- 
tutes ninety per cent of this industry in the United 
States. Railroad ties are treated only by the pressure 
method. The plant needed for such an operation con- 
sists of several long cylinders from six to eight feet 
in diameter and about one hundred to one hundred 
and fifty feet long. The ties to be treated are first 
thoroughly seasoned so as to remove practically all 
available moisture, they are then loaded on cars which 
are run into the cylinder and the doors tightly 
closed. Live steam is turned into the cylinders under 
high pressure and after five or six hours the steam 
and air is pumped out and a vacuum is created 
which draws air and moisture from the cells of the 
timber. The preservative, which may be either creo- 
sote or zine chloride, is forced into the cylinder and 
maintained under approximately one hundred pounds 
pressure until the required amount has been forced into 
the wood—gauges in the tank show how much has 
been absorbed. Ordinarily for complete impregnation 
twenty-five to thirty pounds of creosote is needed for 
sach railroad tie. This makes the treatment com- 
paratively expensive for it may run up to forty-eight 
cents for each piece, but ordinarily less creosote is used, 
as there is little extra benefit obtained from the extra 
material. The average cost of treating a tie is thirty- 
five cents, while poles and fence posts average one dollar 
and fifty cents and ten cents respectively. 
After removal of the treated timbers from the 
cylinders, test holes are generally bored to learn the 
depth of penetration. Cut surfaces are repainted. as 
