Naval Observatory, Washington, D.C.,
    March 4th, 1878.

    EDITOR ST. NICHOLAS:  The periods (of revolution) of the satellites
    of Mars are as follows,—­Deimus being the outer satellite, and
    Phobus the inner one: 

    Period of Deimus, 30 hours, 18 minutes, 0 seconds.
       " " Phobus, 7 " 39 " 16 "

    These values are very nearly correct, and will be changed in the
    final calculation only a few seconds, if at all.—­Yours truly,

    A. HALL.

* * * * *

The following are extracts from the letters of a young girl now traveling in Europe: 

    Berlin, 1877.

DEAR ST. NICHOLAS:  We were in the Auer Cathedral, Munich, looking down the long nave, when troops of little children, boys and girls, each with a little knapsack strapped between the shoulders, leaving the hands and arms free for play, came hastening in by twos and threes, till the whole church seemed full.  They all knelt down, whispered a few words of prayer, and remained for a brief space, silent and motionless, bowed down in devotion; then they quietly arose and went out.  I shall not soon forget Auer Cathedral with its little worshipers.

We have been settled at Berlin for a month.  Being the residence of the Emperor and Court, it is very gay with balls, theaters, etc., and the streets are bright and lively with fine uniforms, prancing horses, and carriages full of richly dressed ladies, their escorts riding on horseback at the side.  It presents a lively contrast with Munich in these respects, but, as to sunlight, it is a gloomy place.  Thus far we have had only four pleasant days, and on those the sun set between three and four in the afternoon.  Some days we thought it did not rise at all!  We realize now, for the first time, how far north Germany is.

We improved one of our pleasant days by a trip to Potsdam, where is the summer palace of the kings of Prussia.  Here are the rooms of Frederick the Great, just as he arranged them.  His library is chiefly of French books, and fills the shelves, which are everywhere, from floor to ceiling—­upon the doors, even, so that, when they are shut, one feels imprisoned in books!

At the opposite end of the palace are the rooms once occupied by Voltaire.  The walls are covered with painted wood carvings of cats, dogs, parrots, and peacocks, which Frederick caused to be placed there after his quarrel with Voltaire, to express his opinion of the Frenchman’s traits of character.

    Directly under the walls of the palace stands an idle windmill, now
    owned by the Emperor.  The noise of this windmill used to annoy the
    queen, so Frederick sent for the miller and said to him: 

    “We two cannot live so near each other.  One of us must buy the
    property of the other.  Now, will you buy my palace?”