1856.] Silt held in suspension by the Hooghly. 159 



Water at the G aspee Floating Light. 



To unprofessional readers, it may be as well to state that the 

 Gasper Floating Light Vessel is moored a few miles below the 

 South point of Sangor Island and 26 miles to the Northward and 

 Westward of the Outer Floating Light Yessel ; so that the Gasper 

 station is exactly where the last traces of the suspended silt of the 

 river may be supposed to reach ; though as lying upwards of 30 

 miles up amongst the sands which form the Sandheads of the 

 Hooghly, it cannot be said to be, like the Outer Floating Light, 

 almost in the open ocean ; so that it is the fairest available spot at 

 which to form an estimate of how much of the silt is carried down 

 to the sea by the action of the surface water, for there is no doubt 

 some action, probably a returning one, going on at the bottom, of 

 which we are unable to form any estimate. It will be noticed that 

 February is but an average from the preceding months, as no water 

 taken in that month reached me. I am indebted to Mr. Parker, 

 H. C. S., Chief Officer of the Star F. L. for this valuable series. 



The following was the process adopted : 



1. The water was first filtered to obtain the silt as before. 



2. It was then evaporated to dryness and re-dissolved in a small 

 quantity of water to obtain the sulphate of linte, no account being 

 taken of the very little taken up by the water of solution. 



3. Bi-carbonate of ammonia being added to the solution, threw 

 down as carbonate of lime what had existed as a muriate, leaving 

 the magnesia in solution. 



4. The magnesia was precipitated by the phosphate of soda and 

 the Ammonia-phosphate of magnesia was calcined to a red heat and 

 the magnesia deduced from it. 



To check the results again, the filters were burnt and the weight 

 of the whole taken, with due allowance for the weight of ash of 

 the filters. 

 For the solid matter; the net weight after combustion was for 



the 11 months, 17.75 



By the table it is, 18.05 



Difference only,.. 00,30 

 which is too small to notice being only .03 or three-hundredths of a 

 grain per month. 



