1887.] S. R/. Elson — On the Density of Surface Sea-wafer, SfC. 15 



Lastly, we have the case of a forest artificially produced by irriga- 

 tion (during the two driest months of the year) in a region so dry 

 that cultivation is rendered possible only by irrigation. Seventeen 

 years' registers at a station within the forest shew an excess of 6 per 

 cent, over the probable rainfall of that station, as computed from the 

 registers of two stations, one of which is 4 miles, the other 13 miles 

 distant from the forest, and both on the borders of the cultivation. 



The evidence is, then, in kind, not rigorously conclusive, and it 

 must be admitted that in no case has it been guarded by those special 

 precautions which are demanded by strict scientific enquiry. But I 

 have no reason to believe that it is not as trustworthy as observations 

 made under the general supervision of intelligent and educated men 

 usually are ; and such as it is, it tends to support and confirm the 

 conclusions drawn a priori from general physical considerations. It 

 justifies, I think, at least, the view I have already expressed elsewhere, 

 namely, that I can no longer regard the long suspected influence of 

 forests on rainfall as a question of equally balanced probabilities. 



II. — On the Changes observed in the Density of the Surface Sea-water, coin- 

 cident with, and due to Aerial Disturb ances^ and consequent Alteration 

 of Baric Pressure over adjacent Sea Areas : and on the Usefulness of a 

 more exact Measurement of the Specific Gravity of Sea-water : more 

 especially with Reference to the Waters near, and ahouty the Hooghly 

 Biver Pilot Station. — By Samuel R. Elson. 



[Received March ; — Read February 2nd, 1887.] 



(With PL IX.) 



In a work which I published some years since, entitled ' The 

 Hooghly Sandheads Sailing Directory,' on the strength of observations 

 made with a small glass instrument, the stem of which was graduated 

 to two thousandths only, I asserted, that the sea water at the Hooghly 

 River Pilot Station contained more salt at low water than it did at high 

 water. But this seeming paradox requires some slight modification, for, 

 I have since then found, with a soda-water bottle hydrometer, which 

 readily weighs the sixteenth of a thousandth of salt in the water, that, 

 in every case, on the least tendency of the sea thereabouts to set to the 

 westward, in response, as I suppose, to aerial disturbances which lessen 

 the baric pressure over the sea area to the southward, the water shows 

 at once a decrease in salinity, consequently, the relative degree of saltness 



