In a letter received from Samuel H. Jack, C. E.: of Cecil county, Md., he states he has made careful observations to determine the true meridian for that place, and has found it to differ from what it should be by E. Loomis' map of magnetic variation in the United States. He made his observations as directed hy Prof. Gillespie's work on surveying, and found the magnetic variation to be 3 30. This is the same as that set down in the map referred to, in ]840; and as the variation at that place has amounted to 3 a year for thirty years, it should be 4 21' now. In making surveys, Mr. Jack should do so upon a meridian established by his own observations; but in making public reports of his labors, he should state this clearly, and the difference between it and the map in question.