544 



NA TURE 



[April 9, 1885 



The maximum, minimum, and mean of all the density ob- 

 servations at each station, together with the number of cases 

 which give the mean, are tabulated (Table III.). The tempera- 

 ture observations made on the monthly trips have not been 

 alluded to here ; they are intended to form a separate paper. 



Table III. — Variations in Density 



Mini 



I. Alloa 



II. Kincardine ... 



III. Hen &Chicke 



IV. Blackness ... 

 V. Inchgarvie ... 



VI. Oxcar 



VII. Inchkeith ... 



Ill 



IX 



X 



XI. OffFiddra ... 



II. May Island... 



99923 



r 00070 

 ,•01084 

 [•01650 



•02566 



Omitting the day of high flood in the 



[•02303* 

 [ '023551 

 [■02472 



[•02382 

 [•02438 



Differences between I 



638 1218 189 2 

 Note. — All the densitit 



1 densities : — 

 f. VI. VII. 



[at i 5 °'56C. 



XII. 



-36 



The change of temperature produces a corresponding change 

 of the density of the water in si/11, which has important bearings 

 on convection currents, and which must also influence the rate 

 of mixture of sea and river water, especially as the temperature 

 of the river water is in winter usually below that of its maximum 

 density. 



An examination of Table III. shows that for maximum and 

 minimum, as well as for mean observed densities, the increase is 

 perfectly continuous until the last station is reached, when the 

 maximum density is only probably greater than that at 

 Station XL, the minimum is certainly less than that at 

 Station VIII., and the mean is less than that at Station IX. 

 It is not easy to find an explanation of this fact, for there is no 

 river nearer than the Tyne at Dunbar, and there are no springs 

 on the Isle of May, the lighthouse keeper being compelled to 

 get all his fresh water carried from Crail. The proximity of the 

 Firth of Tay may possibly account for the observation. 



The difference between the density of the surface-water at 

 high and at low tide is by far the greatest in the upper reaches 

 of the Firth, and decreases more and more gradually. Beyond 

 Inchkeith the difference is little noticed, in fact the water is 

 found to be sometimes denser at a lower state of tide. This 

 effect maybe due to currents which are not taken account of in 

 this preliminary investigation. The rate of change of density 

 with the tide decreases very rapidly at first. At Kincardine the 

 difference between high and low water is 1 in the second 

 place of decimals, or, calling the density of pure water 10,000, 

 it is 100 ; at Inchgarvie it is about 15, at Inchkeith 4, and beyond 

 Inchkeith about 1 ; that is 1 in the fourth decimal place by 

 ordinary notation. 



The density of the surface-water was determined almost 

 daily from August to December at the Scottish Marine Station, 

 (.1 mi >n, but the results were so variable, that dependence 

 cannot be placed upon them as representative of the water in 

 the Firth at that point. The density was always found to be 

 greater at low water, and after some trouble the cause was 

 ascertained. From the station westward for a distance of a 

 mile and a half the shore dries at low water for from quarter to 

 half a mile, and Cramond Island becomes a peninsula, on the 

 west or further side of which the River Almond discharges it- 

 self. At high water there is from half, to one fathom of water 

 between Cramond Island and the coast, and the river, talcing 

 the shorter course, is carried by the ebb tide along the shore, 

 and so reduces the den-ity of the water in the neighbourhood. 



As there was considerable time lost in devising and testing a 

 suitable means of collecting samples of bottom water, the 

 number of reliable cases for consideration is small. 



It may be generally stated that the part of the Firth east of 

 Inchkeith is the region where the difference between the density 

 of surface and bottom water is least, and that the difference 

 decreases steadily towards the May. Towards Alloa, on the 

 other hand, the differences in the density between surface and 

 bottom water are great, but they are greatly influenced by the 

 tide. Table IV. gives details of nineteen comparisons between 

 bottom and surface water. 



At the lien and Chickens Buoy, near Grangemouth, the depth 



is only 5.^ fathoms, but the salinity is very much greater at the 

 bottom than at the surface. The difference is least observed at 

 low water, but as the flood tide sets in, it appears to increase, 

 and then to fall off again as the ebb commences. The only 

 divergence from this rule noticed was on November 11, when 

 the rivers weie all much flooded and the current very rapid. It 

 would appear that these observations confirm the theory that 

 sea water ascends rivers along the jbottom under the opposite 

 current of fresh water. 



« I I 



I I s» II b I ■ 



m I lib M « I 



■ZZ iiii 



£ e8e8c8*i. eBdiJciiit8#<8c3 UUtta a a &<% 



w ^,02; i.-ioz o [»ooa a •=> -so 



Speaking roughly, it appears that waters having an alkalinity 

 under 40 (that is, in which there is less than C04 grammes of 

 carbonic acid as carbonate of lime per litre) have a density 

 under 1 '0200, alkalinities under 25 correspond to densities 

 under i'oioo. The only strikingly anomalous case is that of 

 sample 1S9, a bottom water from off Alloa, when the river was 

 low and very dirty. The density was 1 '00146, the alkalinity 

 47 "9, which usually corresponds to a density of I '024. The 

 presence of sewage in the river might ace Hint for this observa- 

 tion to some extent, but more probably it was due to the pre- 

 sence of particles of calcium carbonate. With an alkalinity 

 between 40 and 50 water has a density between 1*024 anc l ''025 

 as a general rule, and when the alkalinity is over 50 the density 

 is almost invariably over I 025. 



On account of the absence of data for deducing the total salts 

 of estuary water from the density, we cannot reduce the alka- 

 linity to percentage of total salts, and, consequently, it is im- 

 possible to form a correct idea of the difference between bottom 

 and surface alkalinities. as this difference may be entirely due 

 to the different salinity of the water. 



Arrangements have been made for continuing and greatly 

 extending observations on the salinity of the Firth of Forth. 

 Samples of water will be taken by trustworthy observers at high 

 and low water at different points on the Firth, and water- 

 sampling trips, both from Alloa to the May and across the 

 Firth from north to south at various places, will be carried on 

 regularly. 



