940 REPORT—1885. 
the cup, the equation for this case is a little more complicated. With proper 
arrangements, however, the deflection can be shown to be proportional to the 
current strength. 
In the instruments constructed for practical use the mercury cup and canal are 
so made, on the unspillable ink-bottle principle, as to preserve the mercury in case 
ot being knocked over or even inverted. 
12. On the Physical Conditions of Water in Estuaries.! 
By Hues Roserr Mintz, B.Sc., F.R.S.L., F.C.8. 
Observations have been made on the estuaries of several of the most important 
Scottish rivers, of the temperature and density of the water at various posi- 
tions. 
The temperature was observed by means of Negretti & Zambra’s Standard 
Deep-Sea Thermometer, in a special frame devised in order to permit of the 
instrument being worked in shallow water and in places where the current is 
rapid. A modification of the slip water-bottle for the same conditions was made 
and found to work admirably. 
Estuaries were found to be capable of division into three classes :— 
1. Those in which all the salt water is withdrawn from the estuary by the 
ebb-tide, e.g., the Spey. Here the river water freshens the surface of the sur- 
rounding sea, but affects the deep water very slightly. 
2. Those in which the salt water is partially withdrawn, eg., the Tay. Here 
the river is brackish at low tide and the surface water of the surrounding sea is 
slightly freshened. 
3. Those in which salt water always remains, eg., the Forth. Here at the 
river end the density of the water varies with the tide, but is always low ; it rises 
rapidly in the first twelve miles and thereafter, proceeding seaward, it rises very 
gradually. The density of surface and bottom water approaches coincidence as 
the sea is neared, and the merging of the estuary into the sea is marked by a very 
slight decrease of density throughout the entire depth. There is no seasonal varia- 
tion of density, but during a flood the freshening is perceptible all along the line 
in decreasing amount. 
Temperature has a continually diminishing annual range as the sea is approached. 
In summer it falls, and in winter it rises, at first rapidly and then gradually from 
river to sea. The mean annual temperature at each point is the same (47°5). 
Bom water is colder in summer and slightly warmer in winter than that at the 
surface. 
13. Further Experiments in Photo-Electricity. By Professor Mrxcun. 
14. On the Formation of a Pure Spectrum by Newton. 
By G. Grirrira, M.A. 
In English text-books it is generally stated that Wollaston was the first to 
observe a pure spectrum, and that Newton did not know how to form one. 
The author referred to several statements of this character (Professor W. Allen 
Miller, ‘Chemical Physics, 3rd edition, p. 161; Professor Roscoe, ‘ Spectrum 
Analysis,’ 1869, p. 22; see also Parkinson’s ‘ Optics,’ 1859, pp. 148-45.) 
These accounts are probably to be traced to a passage in Sir David Brewster’s 
‘Life of Newton’ (vol. i. p. 117 of the large edition, or p. 58 of the new and 
revised edition, 1875). ‘Had Newton received upon his prism a beam of light 
transmitted through a very narrow aperture, he would have anticipated Wollaston 
and Fraunhofer in their fine discovery of the lines in the prismatic spectrum.’ In 
a previous passage Brewster has described how Newton first formed a spectrum by 
using light which had passed throuch a round hole in a shutter. 
* Published in ewtenso in the Scottish Geographical Magazine, Vol. II. pt. i. 
