RHIZOPHORA MANGLE 



103 



was making its way up the estuary beneath the down-flowing fresher 

 surface water, but the differences in temperature were very slight. 

 During the two days that we lay in quarantine off Puna the density 

 of the surface water ranged between 1*004 and 1*016, being saltest 

 after the tide had been running up for a while. Some time after 

 the tide had commenced to " flow," the whole mass of the river 

 water turned up-stream. The result of this delay in the backing of 

 the water was that the duration of the up-going tide was shorter 

 than that of the ebbing tide. This contrast increased with the 

 ascent of the estuary, so that at Guayaquil, forty miles from the 

 sea, the reversal of the downward current occupied only a short 

 time. 



The undercurrent of sea- water ascending an estuary is a common 

 feature in the regime of a river, and it is one that must have a 

 definite relation with the stations of plants growing at the water- 

 side. With this feature is doubtless to be connected the curious 

 fact that the water may continue to rise after the ebb has begun at 

 the surface. It is apparent that in a tidal estuary a multitude of 

 lines of inquiry offer themselves to an investigator with the 

 hydrometer. 



In some cases, however, valuable indications may be obtained 

 from differences in temperature. When one finds in an estuary that 

 the water at the bottom is warmer by some degrees than the water 

 at the surface, the existence of an undercurrent of sea- water may be 

 surmised. Suspecting that there was an up-current of sea- water 

 below the surface fresh- water of the Black River estuary in Jamaica, 

 I made some observations, at noon in the month of January, in the 

 main stream between the bridge opposite the town and the junction 

 of the Salt Springs River, a mile further up, the depth varying from 

 three to three and a half fathoms. Having first ascertained that the 

 temperature of the sea was about six degrees (Fahrenheit) warmer 

 than that of the river, the sea being 80° and the river 74°, I proceeded 

 to compare the surface and bottom temperatures of the river. A 

 number of observations made in the part of the estuary above stated 

 gave closely similar results, which I may sum up in the remark that 

 whilst the downward surface-current of fresh- water (density 1*001) 

 had a temperature of about 74°, the water on the river bottom 

 showed a temperature of 78° F. Here it was evident that the salt 

 water was ascending the river beneath descending fresh-water, and 

 that the influence of the dissolved salts in the sea-water was more 

 than sufficient to counteract the decrease in density due to its higher 

 temperature. Within the mouth of the Salt Springs branch of the 

 estuary the difference between the surface and bottom temperatures 

 was only two degrees, and a little higher up there was none at all. 



The complicated Problems offered in the Thermometric 



AND HYDROMETRIC INVESTIGATION OF A TlDAL ESTUARY. A multi- 

 plicity of considerations arise when we regard the influence on plant 

 stations of the greatly varying conditions of temperature and salinity 

 in different parts of a tidal estuary; and their complexity deepens 

 when we contrast the regimes of different seasons. The whole 

 subject of the economy of an estuary, as revealed by the thermometer 



