"From the moment they had left the Earth, their own weight, and that of the Projectile and the objects therein contained, had been undergoing a progressive diminution. . . . Of course, it is quite clear, that this decrease could not be indicated by an ordinary scales, as the weight to balance the object would have lost precisely as much as the object itself. But a spring balance, for instance, in which the tension of the coil is independent of attraction, would have readily given the exact equivalent of the loss."