8 



DR HUGH ROBERT MILL ON THE 



above figures, it appears that the limiting velocity does not much exceed 10 feet per 

 second for any depth, supposing the line to hang perpendicularly. In cases when the 

 roughness of the sea prevented the impact of the messengers from being heard or felt, it 

 was accordingly customary to allow one minute as sufficient for the descent of the 

 messengers — the depth, practically, never exceeding 100 fathoms. 



As bearing on the loss of time due to the travelling of vibrations up the line, the 

 forty-two observations of the rate of fall for 10 fathoms were classified as in Table II., 

 which shows that the limiting value was reached almost immediately. 



Table II. — Time occupied by Brass Weight in Falling Ten Fathoms through Water. 



Distance of Fall. 

 Fathoms. 



Approximate 



Depth of the Run 



of 10 Fathoms. 



Fathoms. 



Number 



of 

 Cases. 



Apparent Time 



occupied in Falling. 



Seconds per 10 



Fathoms. 



Mean. 



10 



to 15 

 15 to 30 

 30 to 60 

 Over 60 



13 



13 



13 



3 



7-1 

 7-2 

 7-1 

 74 



7-16 



J 



The impact of the messenger, after even a long run in water, was not sufficient to 

 damage the thermometer ; but when a reversing thermometer was used for a surface 

 observation, the shock of a messenger dropped 10 feet or more from the deck would be 

 dangerous. Accordingly the thermometer at the surface was usually reversed by 

 depressing the lever with a boat-hook, or by lowering a messenger along the sounding- 

 line by a piece of twine. 



Much of the credit for the rapid working which was possible on the " Medusa " is due 

 to the skill and alacrity of her crew, and in particular to the constant attention of her 

 skipper, Mr A. Turbyne, whose unfailing good-humour made the work as pleasant as it 

 was expeditious. 



Treatment of Temperature Results. — The thermometer readings were entered in the 

 observation -book exactly as taken, the correction for index-error being afterwards applied 

 in a column left for that purpose. The observation-book had one page devoted to each 

 sounding, printed headings for position, date, hour, state of weather, &c, and a line for 

 the temperature at each fathom down to 10, then for each 2 fathoms to 20, and for each 

 5 fathoms to 110. The figures when corrected were published as already stated. From 

 the corrected figures curves showing the vertical distribution of temperature at each 

 station for each trip were drawn, the abscissae being temperature, 5 millimetres represent- 

 ing one degree Fahrenheit, and the ordinates being depth, 1 millimetre representing 

 1 fathom. All the vertical temperature curves in the plates illustrating this paper are 

 reproduced on this scale, the paper being divided for the sake of clearness into squares 

 of 5 millimetres instead of 1 millimetre. The curves thus drawn fell into a certain 



