GOSSIP FROM AN AMERICAN LADY IN PARIS.--We derive the subjoined pleasant gossip from a young and gifted American lady, at present resident in the French metropolis. We hope to be similarly favored, whenever our fair correspondent shall find leisure from the demands of society to transcribe her fresh 'jottings-down' for our pages: 'WE went to the Hotel des Invalides this morning to see the plans in relief of the fortified towns of France. They are exhibited to the public only during one month in the year. The plans of those cities I have visited interested me particularly. They are so minute that Miss L----, who accompanied us, had no difficulty in finding the country-house near Toulon where she spent some months last year. I was much struck by the plan of Embrun, in Dauphiny, that little town celebrated in both history and romance by the pilgrimages so frequently made to it by devout Catholics. It is strongly fortified and built on a rock of semi-circular form, which rises so perpendicularly on one side from the meadows which lie below, that one would suppose it must have been hewn away by the hand of man. I was much interested, too, by the plan of Mont St. Michel, in Normandy, the celebrated prison, built on a rock, which the tide separates twice a day from the main land, and where the political prisoners condemned after the _émeuté_, in May, 1839, are confined. We were looking at the plan of a fort, on the pedestal of which the name had not been labelled as on the others, when a soldier standing near informed us it was that of Ham, where the nephew of NAPOLEON is now a prisoner. It is in Picardy, which I am told is the most arid and unfruitful province in France. It is built around a quadrangular court, or I should rather say there are three buildings and a terrace which form a hollow square. The prince, who is said to be a remarkably good horseman, is allowed to ride on this terrace, which is not shaded by trees; indeed, if this plan be correct, which I suppose it must be, as it has just been made, there is but one tree within the precincts of the fortress.... In the evening, my friend, Madame D---- took me to the house of the Polish Princess CZARTORYSKI. Prince ADAM CZARTORYSKI is from his birth, his wealth, and his character, one of the most illustrious of the Polish refugees. His life has been checkered by every extreme of good or ill. In early youth, he served in the army of the republic of Poland. He was at one time the captive of CATHERINE II., and in 1831 was at the head of the provisional government of Poland. After the total defeat of the Polish army, Prince and Princess CZARTORYSKI with some difficulty made their escape, with their three children, the youngest of whom was not a year old. The Princess had not even a change of linen with her, and no time to collect her jewels, which were brought to her after she had reached the Austrian frontier, by a soldier, whom she had never seen before, and who refused to tell his name.