PARA RUBBER. 55 



of scrap rubber ; it is occasionally used on estates for piercing the 

 stem or newly-formed cortical tissue to see if the latex is abundant. 

 The cutting device is mounted at the other end of the handle and 

 consists of a haft or stem with a hollow wedge or triangular-shaped 

 cutting portion at the apex. This knife was one of the first to be 

 placed on the market, and a detailed account of it is given in the 

 India Rubber Journal of February, 1904. 



Bowman's and Northway's Knives. 



These knives have been continually used in the experiments 

 at Peradeniya and Henaratgoda, and in response to suggestions 

 the originals have been slightly modified in order to be of use in 

 any of the numerous systems of tapping, and to still further econo- 

 mize in the removal of the cortical tissues. There are three knives 

 in all : No. 1 for making the original groove, No. 2 for re-opening 

 the lower surface of the wound, and No. 3 for pricking the latex 

 tubes in the area of the wound response without removal of any 

 cortical tissue. These knives are shown in the accompanying illus- 

 trations on Plate 9, .Series A. 



Knife No. 1 is provided with a two-edged guide, which on pressing 

 against the bark cuts the tissue and defines the area to be cut 

 away by the knife edge behind it : by this means the original 

 groove shows clean cut surfaces above and below. It is used much 

 like a plane, the head being suitably adjusted to shave the bark 

 gradually ; as soon as the proper depth is readied the bark is 

 of a white colour and becomes lighter and lighter the nearer one 

 gets to the cambium, so that by practice it is possible to tell almost 

 correctly when the right depth has been cut. 



Young trees are more difficult to cut to the correct depth than 

 old ones, as the latex-bearing tissues below the bark and next to the 

 cambium are very thin indeed ; it is therefore advisable to mark ligb.1 ly 

 with No. 1 and reach the correct depth gradually with a few tappings 

 with No. 2 in the manner described below for cutting deeper. 



Knife No. 2 in its improved form is very ingenious. The cutting 

 part consists of three surfaces, a narrow basal one along which a 

 spring blade is inserted, and two side surfaces at right angles 

 to the basal one. When the flexible spring blade is inserted 

 there are two small cutting edges available, one to use when cut- 

 ting from right to left and one for use from left to right. Several 

 of the No. 2 knives are only provided with one angular cutting 

 surface. By this means only a very thin layer of cortical or bark 

 tissue is removed during each paring operation, the removed sub- 

 stance being so small that it takes quite 30 parings to remove one 

 inch of tissue. This is a most important point, as the trees are made 

 to last considerably over one year instead of only 3 to 6 months. 

 This knife is used only for paring off the lower edge of the grooves 

 originally made, and when cutting should be held so as not to make 



