Once a thing has been done, however difficult or dangerous it may be, it can be done again. Take, for example, the example, the exploration of space. On April 12th, 1961, Major Gagarin of the USSR circled the earth in Vostok I; on May 5th of the same year Commander Shepherd of the USA attained a height of 117 miles in a space capsule; and on August 6th Major Titov made seventeen orbits of the earth in Vostok II.
