produced by a Fluid in Motion. 113 



two thermometers, reading accurately together, were employed, 

 and they were read by means of a lens. I was unable to detect 

 any difference of temperature between the liquid near the sides 

 of the vessel and that (in the same horizontal plane) in the cen- 

 tre ; neither did I find that the liquid soon acquired a uniform 

 temperature. Thus, after heating for some time, the liquid near 

 the top had a temperature of 19° C, that midway between the 

 top and bottom of 23° C, and the liquid at the bottom of 40 o, 5 C. 

 In another experiment, in which the heating had been continued 

 for a longer time, the temperature was taken in successive in- 

 tervals of space — the entire depth of the liquid (175 millims.) 

 being divided into seven layers (as we may call them), each 25 

 millims. thick. The following were the temperatures, begin- 

 ning at 25 millims. below the surface : — 



o 



First (uppermost) layer . . . = 30 C. 



Second layer =30 



Third „ =36 



Fourth „ = 44 



Fifth „ =46 



Sixth (lowermost) layer . . = 46 



It is thus seen that a uniform temperature was by no means 

 rapidly acquired throughout the liquid mass. 



Now, at the commencement of the experiment described above 

 (p. Ill), the oil-sphere floats in static equilibrium ; that is to say, 

 it displaces a quantity of liquid precisely equal to its own weight, 

 in a medium of uniform temperature. On heating the liquid, it 

 soon happens that the layer in contact with the under surface of 

 the oil-sphere becomes hotter, and consequently less dense, than 

 the layer in which the upper part of the sphere floats ; a part 

 of the sphere is now surrounded by a medium of its own density, 

 while the other part is surrounded by a medium of less than its 

 own density. The densities, of the alcohol-and-water mixture 

 and of the oil, do not decrease pari passu for equal increments 

 of heat ; and the oil does not receive heat so readily as the 

 mixture, for it was sometimes found to be as much as 6° C. 

 lower than the layer of liquid in which it floated. Gravity 

 can now act upon the lower portion of the oil; and the con- 

 sequence is that the sphere becomes an ellipsoid with its major 

 axis in the direction of the force which has produced the change — 

 that is to say, parallel to a plumbline. The upper part of the 

 ellipsoid is still unacted upon by gravity, while the lower part 

 has now reached a deeper and consequently warmer and less 

 dense layer ; hence gravity exercises greater force upon it ; and 

 there is nothing to resist this but the cohesion of the oil-particles. 

 Contraction now commences at the point of greatest strain ; and 



Phil. Mag. S. 4. Vol. 33. No. 221. Feb. 1867. I 



