It may be well to mention a few other properties of rock that bear on road-building, though they will not be discussed here. There are some rocks, such as limestones, that are hygroscopic, or possess the power of absorbing moisture from the air, and in dry climates such rocks are distinctly valuable, as the cementation of rock dust is in a large measure dependent for its full development on the presence of water. The degree to which a rock absorbs water may also be important, for in cold climates this to some extent determines the liability of a rock to fracture by freezing. It is not so important, however, as the absorptive power of the road itself, for if a road holds much water the destruction wrought by frost is very great. This trouble is generally due to faulty construction rather than to the material. The density or weight of a rock is also considered of importance, as the heavier the rock the better it stays in place and the better it resists the action of wind and rain. Only a few of the properties of rock important to road builders have been considered, but if these are borne in mind when a material is to be selected better results are sure to be obtained. In selecting a road material the conditions to which it is to be subjected should first be considered. These are principally the annual rainfall, the average winter temperature, the character of prevailing winds, the grades, and the volume and character of the traffic that is to pass over the road. The climatic conditions are readily obtained from the Weather Bureau, and a satisfactory record of the volume and character of the traffic can be made by any competent person living in view of the road. In France the measuring of traffic has received a great deal of attention, and a census is kept for all the national highways. The traffic there is rated and reduced to units in the following manner: A horse hauling a public vehicle or cart loaded with produce or merchandise is considered as the unit of traffic. Each horse hauling an empty cart or private carriage counts as one-half unit; each horse, cow, or ox, unharnessed, and each saddle horse, one-fifth unit; each small animal (sheep, goat, or hog), one-thirtieth unit. A record is made of the traffic every thirteenth day throughout the year, and an average taken to determine its mean amount. Some such general method of classifying traffic in units is desirable, as it permits the traffic of a road to be expressed in one number.