2138. To circulate water in ascending and descending pipes, it is necessary to have a close or covered boiler. In this case a pipe may, at any distance from the boiler, rise to any height, and descend again; but it must neither rise twice, nor dip after leaving the boiler. Where it is necessary to raise it, there should be an open pipe inserted at its extremity, of a height equal to the rise. In heating hothouses, advantage may be taken of this circumstance to carry the hot water over doorways, as fig. 632.