16 PHENOMENON. 
in the circumftances laft mentioned ; and therefore a {mall 
impulfive force, fuch as that communicated by {winging 
the veflel backwards and forwards, will be fufficient to 
raife the water into waves, and produce that commotion 
which is defcribed by Dr. Franklin in the experiment al- 
luded to. 
If this reafoning be juft, then, the lefs difference there 
is between the fpecific gravities of the two fluids, the 
greater will the agitation excited in the lower fluid be, and 
vice verfa; and accordingly you will conftantly find this 
tobe the cafe. For if quickfilver be one of the fluids, the 
waves excited in it will be but very fmall, if water and 
oil be ufed, the waves on the water will be much greater ; 
but if fpirits and oil, of nearly equal fpecific gravities, be 
made ufe of, the commotion excited in the lower fluid will 
be very confiderable indeed. 
Hence we fee the reafon why the motion given toa mug 
of cyder or beer, after having ftood before the fire to 
warm, by {winging itbackwards and forwards in the hand, 
as the common cuftom is in the winter, before one drinks, 
will fo effetually mix the cold and warm parts of the li- 
quor together, which before occupied different places in 
the mug. 
The placid appearance of the upper furface of the oil, 
in the above experiment, is no doubt to be attributed, 2 
part, to the tenacity, or glutinous confiftance of that fluid; 
but the chief caufe undoubtedly is the great difference 
between its {pecific gravity and that of the air—the fluid 
in which it is immerfed. For if oil be made the lower 
fluid, by ufing with it any other fluid of lefs {pecific gra- 
vity, it will, notwithftanding the tenacity of its particles, 
while any confiderable degree of fluidity remains, be af- 
fe€tedin the fame manner as any other fluid in like:cir- 
cumftances, 
An 
