=48. Depth of samples.= The general rule is that the depth of soil samples is determined by the layer to which the roots penetrate. The practice is to remove the air-dried surface in which no roots are found, and to take a sample to the proper depth. This method is open to some objection, as the actively absorbing root surfaces are often localized. There is no practical way of taking account of this as yet, except in the case of deep-rooted xerophytes and woody plants. It is practicable to determine the location of the active root area of a particular plant and hence the water-content of the soil layer, but in most formations, roots penetrate to such different depths that a sample which includes the greater part of the distance concerned is satisfactory. Some knowledge of the soil of a formation is also necessary, since shallow soils do not require as deep samples as others. The same is true of shaded soils without reference to their depth, and, in large measure, of soils supplied with telluric water. In all cases, it is highly desirable to have numerous control-samples at different depths. The normal cores are 12 or 15 inches; control-samples are taken every 5 inches to the depth desired, and in some cases 3–inch sections are made. It has been found a great saving of time to combine these methods. A 5–inch sample is taken and placed in one can, then a second one, and a third in like manner. In this way the water-content of each 5–inch layer is determined, and from the combined weight the total content is readily ascertained. =49. Check and control instruments.= A number of instruments throw much light upon the general relations of soil water. The rain-gauge, or ombrometer, measures the periodical replenishment of the water supply, and has a direct bearing upon seasonal variation. The atmometer affords a clue to the daily decrease of water by evaporation, and thus supplements the rain-gauge. The run-off gauge enables one to establish a direct connection between water-content and the slope and character of the surface. The amount and rapidity of absorption are determined by means of a simple instrument termed a rhoptometer. The gravitation water of a soil is ascertained by a hizometer, and some clue to the hygroscopic and capillary water may be obtained by an artificial osmotic cell. All of these are of importance because they serve to explain the water-content of a particular soil with especial reference to the other factors of the habitat. It is evident that none of them can actually be used in exact determinations of the amount of water, and they will be considered under the factors with which, they are more immediately concerned. _Physical and Physiological Water_