The Isthmus in 1904 19 



rings of a semi-spherical section around each 

 plant. They were kept filled with water and of 

 course contained many larvae of both Aedes calopus 

 and Anopheles as well as algs, vegetable debris, 

 etc. 



The ditches near the wards were lined with field 

 stone; the spaces between the stones were filled 

 with earth and grass which retarded the current of 

 water. These ditches discharged onto the grass- 

 covered, clayey soils, and pools remained there, 

 distributed over a large area. 



The natural topography of the hill gave a fairly 

 steep slope with a more gently sloping bench below 

 it. The highest part of the hill is about six hun- 

 dred feet above sea level. The water absorbed by 

 the upper part of the hill came to the surface at a 

 lower level on a clay-Uke formation covered with 

 grass, making the hillside resemble a bog in many 

 places. The outcrop of water, which oozed 

 slowly from the ground, often followed the contour 

 of the hill for a considerable distance. Conse- 

 quently the- area immediately below that contour 

 was wet almost continually. 



There was a cow pasture on the top of the hill, 

 and throughout the rainy season the hoof-prints 

 remained wet or full of water. Ditches of varying 

 widths ran down the slope, and the water spread 



