41f. 



Part III. — Eleventh Annual Report 



at the beginning of June and the beginning of July, until the maximum 

 is reached at the end of that month and the first days of August. A 

 marked rise of temperature occurs in November, as in 1891, and a minimum 

 is reached shortly before the end of the year. 



The surface and bottom temperatures were the same as the annual mean 

 at the end of April, and again in the second week of October, exactly as 

 in the case of the other West ("oast stations except Brodick. 



6.— PHYSICAL OBSERVATIONS MADE ON BOARD THE 

 ' GARLAND,' 1891-92. 



The observations of pressure, temperature, and transparency made on 

 board the " Garland " in 1891 at nine Stations in the Firth of Forth are 

 printed in Tables on pages 439-41. Similar observations and salinity 

 records in addition, taken during 1892 at ten Firth of Forth Stations 

 and at five Stations in St Andrews Bay, will be found on pages 470-77. 



A map showing the exact position of these Stations was inserted in the 

 Ninth Annual Report. Observations at Station X. are published for the 

 first time this year. It is situated off Limekilns, above Queensferry. 



The ' Garland ' observations were made once a month at each station 

 in the Firth of Forth, and also monthly, except in July, August, and 

 September, at Stations in St Andrews Bay. They are carried out with 

 great care and skill, and afford very reliable and important material for 

 the full discussion of the Physical conditions of the Firth of Forth. This 

 is too extensive an investigation to be dealt with in the present Report, 

 hut special attention may be paid to one set of observations of unique 

 interest. 



The observations on the transparency of the waters of the Firth of 

 Fjrth were discussed for the first time in the Ninth Annual Report. 

 The method of determining it is by sinking a white enamelled disc, 3 feet 

 in diameter, until it just disappears from view. A note is made of the 

 number of fathoms the disc is below the surface when this happens. 



The clearness with which objects can be seen through the water seems 

 a mutter of capital importance for the fishermen using nets or even 

 lines , and consequently special attention is again directed to the results 

 of the " Garland " observations. 



The depth of visibility depends on the amount of light, determined by 

 the height of the sun, so by the season and hour of observation, on the 

 cloudiness of the sky, on the transparency of the air, as well as on the 

 clearness of the water. The first named factors necessarily have a large 

 influence in determining the depth at which the white disc vanishes as 

 long as the water is of fairly constant clearness ; and the seasonal 

 variations, shown in the accompanying tables, are almost wholly due to 

 these factors in the case of the tSt Andrews Bay Stations, and those 

 towards the mouth of the Firth of Forth. But the clearness of the water 

 itself varies. In winter-time the flooded rivers often bring down great 

 quantities of suspended matter, and partly cause the diminished depth of 

 visibility at the Upper Forth Stations. This sediment, which obscures 

 the water higher up the river, makes the transparency diminish as 

 the Firth and Estuary are ascended. Again at the west end of Station 

 IV., where the Esk and the Water of Leith both have some influence, and 

 wheie the water is usually very muddy, there is very little seasonal 

 variation ; whereas at Station IX., which is E.S.E. of the May Island, 

 the seasonal variations are well marked, as the mean readings for four 



