Where the ground is firm, no other support is needed for the tubes than to tamp the earth solidly about them. In order to economize space in the streets, it is customary to lay the tubes one above the other; and it is very convenient, although not necessary, to separate them by cast-iron saddle brackets. Such an arrangement has to be frequently departed from in order to overcome obstructions in the streets and to get through narrow passages. At all low points in a tube line, traps are provided to catch any moisture that may accumulate. These traps are made accessible for frequent inspection by means of man-holes or otherwise. The tube is usually laid about three feet below the pavement. This distance has frequently to be varied, but it never becomes so small as to render the tubes liable to injury from heavy trucks passing over the pavement. CHAPTER IV. FACTS AND GENERAL INFORMATION RELATING TO PNEUMATIC TUBES. We will now discuss, in an elementary way, the theory of pneumatic tubes, in order to understand more clearly their _modus operandi_ and the principles upon which they should be constructed. Let us begin with the definition of a pneumatic tube. =Definitions.=—A pneumatic tube is a tube containing air. This is perhaps the broadest and most comprehensive definition that can be given, but we usually associate with the idea of a pneumatic tube the use to which it is put. If we were to embody this idea in our definition we might define a pneumatic tube as a tube through which material is sent by means of a current of air. This is still a very broad definition, including all kinds of material for transportation, for every conceivable purpose. It places no limit upon the dimensions of the tube nor the manner of its operation. This definition would include the toy commonly known as a putty-blower, and the pneumatic gun. These instruments are not usually pictured in our minds when we hear or see the term pneumatic tube used. Instead of these, we think of the brass tubes that we have seen in the large retail stores in some of our cities for conveying cash from the various counters to the centrally located cashier’s desk. Again narrowing our definition to conform more nearly with the mental picture presented, we will define a pneumatic tube as a long tube for the purpose of transporting material in carriers by means of a current of air in the tube. This, like all definitions, is not entirely satisfactory, if we examine it critically, but it will answer our present purpose.