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One of the isothermal surfaces will be a sphere S surrounding the sink, which, however, will not be at the centre of the sphere, but so situated that the source, sink, and centre are in line, and that the radius of the sphere is a mean proportional between the distances of the source and sink from the centre.

Define ratio, proportion, mean proportional, third proportional, fourth proportional.

Find a mean proportional between 4 and 16; 18 and 50; and 3.

The second of three numbers is a mean proportional between the other two.

If 4 quantities are in proportion and the second is a mean proportional between the third and fourth, prove that the third will be a mean prop. between the first and second.