HIGHWAY CONSTRUCTION 103 



(8) It is moderate in first cost. 



(9) It is not disagreeably noisy. 



Defects. The principal objections to wood pavement are: 



(1) It is difficult to cleanse. 



(2) IJnder certain conditions of the atmosphere it becomes 

 greasy and very unsafe fci? horses. 



(3) It is not easy to open for the purpose of gaining access to 

 underground pipes, it being necessary to remove rather a large surface 

 for this purpose, which has to be left a little time after being repaired 

 before traffic is again allowed upon it. 



(4) It is absorbent of moisture. 



(5) It is claimed by many that wood pavements are unhealthy. 

 Quality of Wood. The question as to which of the various kinds 



of wood available is the most durable and economical, has not 

 been satisfactorily determined. Many varieties have been tried. In 

 England, preference is given to Baltic fir, yellow pine, and Swedish 

 yellow deal. In the United States the variety most used (on account 

 of its abundance and cheapness) is cedar ; but yellow pine, tamarack, 

 and mesquite have also been used to a limited extent, and cypress arid 

 juniper are being largely used in some of the Southern States. 



Hardwoods, such as oak, etc., do not make the best pavements, as 

 such woods become slippery. The softer, close-grained woods, such 

 as cedar and pine, wear better and give good foothold. 



The wood employed should be sound and seasoned, free from sap, 

 shakes, and knots. Defective blocks laid in the pavement will quickly 

 cause holes in the surface, and the adjoining blocks will suffer under 

 wear, the whole surface becoming bumpy. 



Chemical Treatment of Wood. The great enemy of all wood 

 pavements is decay, induced by the action of the air and water. Wood 

 is porous, absorbs moisture, and thus hastens its own destruction. 

 Many processes have been invented to overcome this defect. The 

 most popular processes at present a*re creosoting and modifications of 

 the same, known as the "creo-resinate" and "kreodine" processes. 

 These consist of creosote mixed with various chemicals which are 

 supposed to add to the preserving qualities of the creosote. 



Creosoting. This process consists in impregnating the wood with 

 the oil of tar, called creosote, from which the ammonia has been ex- 

 pelled, the effect being to coagulate the albumen and thereby prevent 



