MATERIALS. 131 



Roman cement soon loses its setting properties when exposed 

 to the air ; it should therefore be kept in the casks in which it 

 is supplied, and in a dry place, until required for use. Owing to 

 the rapidity with which it sets, only small quantities should be 

 gauged as required for immediate use. 



Medina Cement has already been referred to (p. 122). It is 

 of a light yellow-brown colour, and is made from septaria found 

 in Hampshire and in the Isle of Wight. In its properties and 

 uses it much resembles ordinary Roman cement. 



The three cements which have now been described, namely, 

 Portland, Roman, and Medina, are those commonly used in 

 harbour- works; there are, however, some others which are 

 occasionally used, but it is not necessary to further refer to 

 them here. Full information respecting them may be found on 

 reference to " Notes on Building Construction," Part III. (Long- 

 mans & Co.), or to " Limes, Cement, etc." (Lockwood & Co.). 



The use of lime and puzzuolana in sea-works, as commonly 

 adopted by French and Italian engineers, was noticed when 

 treating of concrete, in Section III. of this chapter. 



SECTION V. TIMBER. 



Various kinds of timber used in sea- works Destruction of timber by sea-worms, etc. 

 Teredo navalisLimnoria terebrans Chelura terebrans Methods of pre- 

 serving timber Greenheart, sneezewood, and jarrah repel sea-worms 

 Creosoted Baltic red-wood, oak, and teak useful in some localities Pitch pine 

 quickly attacked by sea-worms Greenheart and jarrah liable to split 

 American rock elm useful for fendering Blaok iron wood eaten freely by 

 Teredo navalis. 



The different kinds of timber which are more or less commonly 

 used in sea-works are as follows : 

 Baltic red- wood (Pinus sylvestris). 

 Pitch pine (Pinus rigida). 

 Oregon pine (Abies Douglaisi). 

 English oak (Quercus robur). 

 American oak (Quercus alba). 

 Teak (Tectona grandis). 

 Greenheart (Nectandra rodicei). 

 Jarrah (Eucalyptus marginata). 

 American rock elm (Ulmos racemosa). 

 In addition to these, black ironwood (Olea laurifolia) and 



