PARA RUBBER, 51 



be detected many years after it has been made. Such knobs and 

 scars are not due to " canker," and the establishment of a smooth 

 surface on such trees without cutting into the wood is practically 

 an impossibility. 



The tapping of irregular surfaces requires special consideration; 

 but it may be stated that in no case should the woody protuberances 

 be excised ; the incisions should, if possible, be made above or below 

 all woody warts, and the latter allowed to work themselves out in 

 their own way and time. In such cases the zigzag method of tap- 

 ping (sec next chapter) can often be adopted with advantage. 



Tapping Knives. 



The various methods of tapping now in vogue are often associa- 

 ted with the use of a particular knife or series of knives, and it is 

 therefore necessary to first consider the knives which are commonly 

 used and the general requirements of such implements. 

 Requisites of a Good Tapping Knife. 



There are several points which should be borne in mind by those 

 who desire to effect improvements in tapping knives or to invent 

 new ones. 



The first requisite is that the cutting surfaces shall be such as to 

 enable the operator to either make an even clean cut or to excise the 

 cortical tissues without dragging the cells or clogging the knife. 

 Several friends have shown me instruments which are best 

 described as surgical scrapers, planes, and closed knives ; in each case 

 the idea was to scrape away a thin film of the cortical tissue, but in 

 every instance the operation dragged the cortical cells considerably, 

 clogged up the latex tubes, and left an uneven surface along which 

 watery latex would not flow. A clean cut is essential, and for this 

 reason it is doubtful whether the principle of scraping will ever be 

 generally adopted. 



A second point of very great importance is that the knife should, if 

 possible, be provided with some structure which will prevent the 

 cooly from cutting too deep when making the initial excision, and 

 also protect the cambium during subsequent paring operations. In 

 several cases separate knives are used for making the original in- 

 cision and subsequent paring operations ; those used in the latter 

 processes are frequently made so that they can be adjusted before- 

 hand, or they are protected by a fixed or detachable blade. A glance 

 at illustrations on Plates 8, 9, and 10, Series A, will show the 

 appliances referred to. The effect of bad tapping is shown elsewhere. 

 It is a great advantage if the cutting parts can be adjusted with ease 

 and replaced without great expense. 



A third consideration, which should not be lost sight of, is that the 

 knife should be one which can be used in cutting from left to right 

 and right to left from above downwards. Illustrations 7 and 10 

 show knives which can be so used, and also from below upwards if 

 desired. This is a necessary qualification in all tapping methods 

 except the right-hand half-herring-bone and spiral systems ■" 



