Heat and Ventilation. Commercial greenhouses usually are heated by steam, hot-water, or hot-air systems. Hobby greenhouses are often heated by extending the hot-water or hot-air circulating system used for home heating. Separate small hot-water and hot-air units and electric heaters are available for the home greenhouse.
The water circulating through the sand deposits iron oxide, silica or lime between the grains and this "cements" the loose sand into sandstone. Mud is squeezed by the weight of the sediment above until ail the water is pressed out and it becomes shale. This process of changing sediment into rock is called lithification.
In all actual cases of refrigeration the substance which absorbs the rejected heat is circulating water, which becomes more or less warmed by the heat which it takes up. The refrigerating machine accordingly works between two temperatures ; the lower temperature is that at whici heat is taken in from the body which is to be kept cold, and the higher temperature is that at whicl heat is rejected to the circulating water or other substance which absorbs it. In order that this process may go on, an expenditure of mechanical power is necessary.
Heat and Ventilation. Commercial greenhouses usually are heated by steam, hot-water, or hot-air systems. Hobby greenhouses are often heated by extending the hot-water or hot-air circulating system used for home heating. Separate small hot-water and hot-air units and electric heaters are available for the home greenhouse.