A Letter from Humboldt, Praying Release From the Letter Writers. --The venerable Humboldt thus appeals for mercy to a class which was never yet known to spare either sex or age: Berlin, March 15, 1859. Suffering beneath the pressure of a still increasing correspondence, amounting to between sixteen hundred and two thousand communications per annum, and embracing letters, printed pamphlets on matters which I am wholly unacquainted, manuscripts concerning which my opinion is desired, projects for emigration and colonizing, the sending of models, machines and natural curiosities, request of contributions to collections of autographs, efforts to take charge of my domestic concerns, to cheer and enliven me, &c., &c., I must again publicly urge all persons having my welfare at heart to exert their influence in my behalf, that individuals on both continents may no longer busy themselves with my person and make of my house an intelligence office; and that I be allowed, in the declining state of my physical and mental powers, to enjoy some rest and leisure to attend to my duties. May this cry for relief, which I utter with much reluctance and after reflection, not meet an unfriendly interpretation! Alexander von Humboldt.