For more than half a century before the building of the great Pacific railways, engineer officers were engaged in making surveys and explorations in the great unknown country west of the Mississippi River, and the final map of that country was literally covered with a network of trails made by them. Several of these officers lost their lives in such expeditions, while others lived to become more famous as commanders during the great rebellion. Generals Kearney, J.E. Johnston, Pope, Warren, Fremont and Parke, and Colonels Long, Bache, Emory, Whipple, Woodruff and Simpson, Captains Warner, Stansbury, Gunnison and many other officers, generally in their younger days, contributed their quota to the geographical knowledge of the country, and made possible the wonderful network of railways guarded by military posts that has followed their footsteps. Their reports fill twelve large quarto volumes. BOUNDARY AND LAKE SURVEYS. The astronomical location of the boundaries of the several States and Territories, as well as of the United States, is a duty frequently required of the engineer officer, and such a survey between this country and Mexico is now in progress. The entire line of the 49th parallel of latitude from the Lake of the Woods to the Pacific Ocean, which forms our northern boundary, was located a few years ago by a joint commission of English and United States engineers, and monuments were established at short intervals over its entire length. A careful geodetic and hydrographic survey of the Great Northern Lakes, including every harbor upon them and the rivers connecting them, was carried on for many years and was finally completed some ten years ago. Maps and charts of these surveys are published from time to time for use of pilots navigating these waters.