CLYDE SEA AREA. 41 



an intermediate maximum or minimum' occur for a much longer space of time than can 

 be accounted for on the hypothesis of the tidal currents producing the irregular mixture. 

 In the case of c, the minimum temperature at 13 fathoms is 52° "2, almost the same 

 as that of the homothermic water of the Channel at the same time. The observation was 

 made at a point nearly midway between Pladda and Sanda, but close to Sanda the 

 curve (Table IX. No. 6) appeared homothermic at 52°*8. The sharpness of the included 

 minimum between 9 and 14 fathoms in c leaves some room for the speculation that 

 in the space between 5 and 15 fathoms in No. 6 (d) a similar minimum may have escaped 

 attention. At anyrate, this justifies the great caution adopted in connecting the points 

 of observation to form curves (see p. 9). The last example, Table VII. No. 6, of 

 the Eastern division of the Plateau, fig. 8, e, is too wild in its zigzags to be 

 accepted as natural, and it is only introduced to illustrate the necessity for keeping 

 the vessel exactly in the desired position while all the soundings of a serial set are 

 carried out. On this occasion the first sounding was made about 1^ miles to the 

 south of Pladda, in a depth of 26 fathoms, and temperatures taken at 24, 14, and 4 

 fathoms. The second sounding gave observations at 18, 8, and 3 fathoms, and the other 

 points were then filled in. At the end, the extraordinary sequence of temperatures 

 induced me to take another bottom temperature, when it appeared that the depth 

 was only 18 fathoms, and the temperature at 17 fathoms 56°"2, and that the "Medusa" 

 had been drifting toward the land while the observation was being: made, and so comins; 

 into the much warmer shore water. The sea was rough, and the weather dull and rainy, 

 so that landmarks could not be distinguished. The total drift was probably not more 

 than quarter of a mile. 



Hourly Observations on Plateau. — On June 17th, 1887, a series of thirteen hourly 

 observations was made when anchored nearly midway between Sanda and Pladda, 

 Davaar bearing N.W. by N. f N. distant 6|- nautical miles. Observations were 

 commenced at 20 h 0, and carried on until eight on the morning of the 18th. The depth 

 varied from 26 to 27 fathoms, but the deepest water did not correspond with the 

 theoretical hour of full tide, which was 21 h 30, low water occurring at 3 h 0. The tempera- 

 ture of the air was 60° at the time of commencing observations, 59° at midnight, and 

 58° at 2 o'clock in the morning. The thermometer for air- temperature was broken 

 during the observations, but the air was not perceptibly colder after two o'clock. The 

 night was dead calm, with a thick haze shutting out the stars and lighthouses, and 

 thus tending to minimise the cooling due to radiation. 



The changes which occurred in the twelve hours were of a remarkable character. 

 Fig. 9, Plate XXIV., shows the variation of temperature as ascertained hourly at the sur- 

 face, 5 fathoms, 10 fathoms, 15 fathoms, and the bottom. The variation of the mean 

 temperature of the whole slice of water is also shown, and the tidal phase is indicated 

 below the figure. The surface temperature fell gradually but irregularly from 59°0 at 

 20 h to 57° *3 at 8 h 0. A minimum occurred at 23 h 0, another very slight one at 3 h 

 (57°'4), and a much deeper minimum (56 o> 0) at 7 o'clock on the 18th. At 5 fathoms 



VOL. XXXVIII. PART I. (no. 1). F 



