tail requires scrutiny, but with care we see that each feature is being driven away from the head.
We thus realize that though gravity is the principal force with which we have to deal when we are travelling out into space, there are, nevertheless, other forces such as that which acts on the lighter particles of a comet's tail. I hope none of us on the journey will be blown away like the lighter particles.
The facts that the Earth rotates and that it pulls us towards itself by gravity are so familiar to us that we scarcely realize their immense importance, and it may seem strange to you to have spent so much time talking about them. But you remember the old proverb that a cow never knows the value of her tail till she has lost it. If we leave the Earth to take our voyage in space we shall lose both these things. We shall lose the Earth's rotation, and with it the changes from day to night. If the Earth stood fast as our forefathers believed, the Sun and the stars revolving round it, we should retain something of the daily change in our journey: but now we know that it is the heavens which remain still, while the Earth rotates, we must make up our minds to lose the peaceful night with its invitation to sleep. If we sleep at all, we must sleep by broad daylight, for the Sun will shine continuously when once we lose the Earth's shelter. Our watches will tell us the time, but clocks, which are driven by weights, and regulated by pendulums, will be useless when we lose the gravity that pulls the weights down and causes the pendulum to swing. It was Galileo who first found out that a pendulum kept time, and he made the discovery in the cathedral at Pisa when the lamps
