When you are face to face with a difficulty, you are up against a discovery.

Large increases in cost with questionable increases in performance can be tolerated only in race horses and fancy women.

Do not imagine that mathematics is hard and crabbed, and repulsive to common sense. It is merely the etherealization of common sense.

[Of the ether] it is no greater mystery at all events than the shoemakers' wax.

I can never satisfy myself until I can make a mechanical model of a thing. If I can make a mechanical model, I can understand it. As long as I cannot make a mechanical model all the way through I cannot understand.

Fourier is a mathematical poem.

I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind.