76 The Control of Mosquitoes 



period came from a flat depression north of Gatun 

 Dam. In making the channel near the entrance 

 to the lower lock the material was excavated by 

 dredges and passed through pipes onto the area 

 referred to.' The canal charmel at points where 

 the dredges operated was several hundred feet 

 wide and subject to the usual rise and fall of 

 the tide. Previous to the introduction of salt 

 water into this area it produced some Culex but 

 not many Anopheles. It was covered with high 

 grass and in many places with brush. The 

 brush and trees began to die soon after the intro- 

 duction of salt water. As the work of filling 

 progressed, the increasing body of water spread 

 over many acres and this sheet of water, about six 

 inches deep, was hidden by tall grass. The water 

 was deeper farther off shore. The area was more 

 than a mile long and about half a mile wide. 

 Flight observations indicated and fairly proved the 

 direction the influx was taking before it was known 

 by actual inspection of the swamp that this water 

 was the source of thousands of mosquitoes that 

 affected the settlement from half a mile to a mile 

 distant. The water in the grass around the edges 

 of the newly formed lagoon remained nearly fresh 

 and no salt could be tasted along shore. In some 

 ' See Map of Gatun No. 36 



