The Director of the Cambridge Observatory to the President of the Gun Club at Baltimore. CAMBRIDGE, October 7. On the receipt of your favor of the 6th instant, addressed to the Observatory of Cambridge in the name of the members of the Baltimore Gun Club, our staff was immediately called together, and it was judged expedient to reply as follows: The questions which have been proposed to it are these— “1. Is it possible to transmit a projectile up to the moon? “2. What is the exact distance which separates the earth from its satellite? “3. What will be the period of transit of the projectile when endowed with sufficient initial velocity? and, consequently, at what moment ought it to be discharged in order that it may touch the moon at a particular point? “4. At what precise moment will the moon present herself in the most favorable position to be reached by the projectile? “5. What point in the heavens ought the cannon to be aimed at which is intended to discharge the projectile? “6. What place will the moon occupy in the heavens at the moment of the projectile’s departure?” Regarding the first question, “Is it possible to transmit a projectile up to the moon?” Answer.—Yes; provided it possess an initial velocity of 1,200 yards per second; calculations prove that to be sufficient. In proportion as we recede from the earth the action of gravitation diminishes in the inverse ratio of the square of the distance; that is to say, at three times a given distance the action is nine times less. Consequently, the weight of a shot will decrease, and will become reduced to zero at the instant that the attraction of the moon exactly counterpoises that of the earth; that is to say at 47/52 of its passage. At that instant the projectile will have no weight whatever; and, if it passes that point, it will fall into the moon by the sole effect of the lunar attraction. The theoretical possibility of the experiment is therefore absolutely demonstrated; its success must depend upon the power of the engine employed. As to the second question, “What is the exact distance which separates the earth from its satellite?” Answer.—The moon does not describe a circle round the earth, but
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