^95 
grows over this and forms a complete mat over which the rain runs 
without erosion. The air from below keeps this mat very porous, 
and the mat then acts as a filter for future rain. Care must be taken 
to cut down any trees the roots of which will interfere with the pipes. 
In the case of seeps, if the borings made do not give sufficient 
data, two or more holes must be dug and the slope that the water is 
following determined. I'his is sometimes so steep that parallel 
pipes have been laid, lO' apart and 5' deep, to catch completely the 
water, as it is essential that all water should be intercepted. 
With regard to cost, it must be remembered that the cost of 
transportation and labour is exceedingly high on the Isthmus. A 
good labourer receives $1.80 per day U.S. currency. Some work 
would be in inaccessible valleys, and complete roads would have to be 
built. Rock would often have to be transported by train and then 
carried over hills. This transportation has made the work 
considerably more costly than it would be in other places. The 
average cost of the tile put in so far is about 35 cents per foot, about 
IS. 3d. of which is the first cost of the tile. The cost of maintenance 
of open ditches for the year amounted to 25 cents per foot, so that in 
less than a year and a half the work has paid for itself, apart from the 
great reduction in malaria and the great saving in the cost of the 
care of the sick. The work is yet far from complete, and much 
remains to be done. 
From the experience gained here it would appear that 
sub-drainage work is the only practical means, taking into 
consideration the cost of upkeep, for permanent anti-malarial work. 
In a climate such as this, and where Anophelines breed m clear 
running water during all months of the year, the annual expense of 
the open ditch system is prohibitive. 
