386 Hydrometrical Observations. 
tus was secured by a cord of sufficient length to sink it to the required 
depth, and the whole was attached to a tin buoy, letter 6, which floated 
on the surface, its form being such as to produce little resistance to its 
passage through the water. The buoy served not only to preserve the 
yane plate at the same depth, but also indicated its progress through the 
water in a very satisfactory and often interesting manner. 
The plate, sunk at the depth of 50 feet, when acted upon by the force 
of a strong under current, was hurried along, carrying the buoy, which 
floated on the surface, along with it, a circumstance which was ascer- 
tained by the buoy passing the floats thrown out on the water as gauges 
of the velocity and direction of the upper current, one of which is shewn 
atc. The only precaution to be observed in making such observations, 
is to exclude that part of the commencement of the buoy’s course, which 
is more rapid than it ought to be, owing to the effort made by it to over- 
take the plate, which, being sunk first, has been influenced by the velo- 
city of the under current before the buoy has been launched. It is evi- 
dent that, by means of this simple apparatus, we can approximate to the 
direction as well as to the velocity of under currents; but it must be 
kept in view that, in either case, there are several deranging influences 
in operation, which tend to render the results obtained merely rude ap-~ 
proximations to the truth. 
The direction of surface currents may be easily observed by means of 
a string of cork floats. Any change in the direction of the line traced 
by the floats is noted by observations made with the surveying compass 
or the sextant, by an observer stationed in a boat, which is rowed along- 
side of the line marked out. 
The last hydrometrical topic which shall engage our attention, is the 
method of obtaining specimens of water at different depths, with a view 
to ascertain its qualities in regard to the proportion of sea salt which it 
contains, or the quantity of sand or mud held in mechanical suspension. 
The first observations made on this subject, so far as I am aware, were 
those instituted by my father on the River Dee in Aberdeenshire, in the 
summer of the year 1812, when engaged in surveying that river in refe- 
rence to a salmon fishing case.* ‘‘ He observed in the course of his sur- 
vey that the current of the river continued to flow towards the sea with 
as much apparent velocity during flood as during ebb tide, while the sur- 
face of the river rose and fell in a regular manner with the waters of the 
ocean. He was led from these observations to enquire more particularly 
into this phenomenon, and he accordingly had an apparatus prepared, 
under his directions, at Aberdeen, which, in the most satisfactory man- 
ner, shewed the existence of two distinct layers or strata of water; the 
lower stratum consisting of salt or sea water, and the upper one of the 
fresh water of the river, which, from its specific gravity being less, floated 
* Report to the Earl of Aberdeen and the other proprietors of the “ Raik” and 
“Stell” fishings of the River Dee, at Aberdeen, by Robert Stevenson, Civil En- 
gineer. Edinburgh, Feb, 1813. 
ae 
