Different link: http://www.gutenberg.org/etext/5001 If you assume a standard clock, say the second on earth at sea level, you will notice a difference between the second of this clock and the second of a clock on top of Mt. Everest - assuming that these devices are identical. An experiment measuring the speed of light on top of Mt. Everest will give the same result as an experiment measuring the speed of light at sea level. But, now it is obvious that the experiments themselves are being executed at different rates. They're different rates because of how initally synchronized clocks will desynchronize after being subjected to a change altitude - Gravitational Time Dilation.