308 On the use of the Marine Hydrometer. 



great number of specimens were to be tried on the spot, and 

 during the progress of the surveys, it occurred to me, that 

 something on the principle of the hydrometer might be intro- 

 duced, which wouhl facilitate the business ; and this is one of 

 the instruments which I found to answer. It consists merely 

 of a common spirit hydi'ometer-bulb, made so long as just to 

 sink under the bulb in sea-water, and adapted with a very 

 thin scale, so as to give gi'eater sensibility, and measure the 

 different shades of saltness with accuracy. Considerable 

 difficulty was found in adapting this scale, as it must not 

 only be thin, but light, otherwise it tends to overbalance the 

 whole instrument. A thin slip of whalebone or ivory answers 

 sufficiently well. A stem of glass would be desirable, but it 

 is too slender, and liable to be broken. Here is an instru- 

 ment entirely of brass, like the brewers' hydrometer, and will 

 answer very well ; and I have no doubt, that in the hands of 

 instrument-makers, a more finished and correct instrument 

 could be constructed for general use ; and it would be curious 

 to have experiments with such an instrument in different seas. 

 The general specific gravity of sea-water along the shores 

 on the east coast of Scotland, I have found rai-ely to exceed 

 102G. Fresh water being 1000. The use of the instrument 

 was shewn in different waters, and a very small impregnation 

 of salt was visible in fresh water by the rising of the stera. 

 A specimen from Granton Pier at low water was found 1 024, 

 shewing an impregnation of one part of fresh in 13 of salt, 

 and at high water, it was exactly the same, and also the 

 same at the top and bottom : but this is seldom the case at 

 the mouths of rivers and estuaries, the fresh water being 

 found generally floating on the surface, particularly in rivers 

 such as the North Esk in Forfarshire, which, making a rapid 

 and sudden descent into the sea without an intervening 

 estuary of any ex'jent, no time is allowed for the mixture of 

 the waters. I have frequently found the waters there per- 

 fectly fresh on the sui'face, and in the water at 4 or 5 feet 

 deep, the hydrometer mounted nearly to the top of the scale, 

 shewing the entire prevalence of the sea-water at that depth. 

 After their descent into the open sea, the fresh waters float 

 about on the surface for a long time, and are driven in dif- 



