=Pressure and Exhaust Systems.=—We have noticed that pneumatic tubes have not always been operated by compression of the air, but that some of the small tubes used in the telegraph service of European cities have been operated by exhausting the air. The two systems are sometimes distinguished by calling one a pressure system and the other an exhaust system. These terms are very misleading, for an exhaust system is a pressure system. The current of air is kept flowing in a tube by maintaining a difference of pressure at the two ends, and the result is the same whether we raise the pressure at one end above the atmospheric or lower it at the other below the atmospheric. In either case it is pressure that causes the air to flow. It happens that we are living in an atmosphere of about fifteen pounds pressure per square inch, and it is very convenient oftentimes in our computations to take the pressure of the atmosphere as our zero, and reckon all other pressures above and below this. If all our pressure scales read from absolute zero, the pressure of a perfect vacuum, then all this confusion would be avoided. We have not used the absolute zero in our diagram, because all our gauges are graduated with their zero at atmospheric pressure, and it is customary to speak of pressures above and below the atmospheric. It is very natural to ask the question, why are tubes sometimes operated by compressing the air and at other times by exhausting it? We answer by saying that it is usually a question of simplicity and convenience that determines which system shall be used. Some of the cash systems in the stores use compressed air in the out-going tubes from the cashier’s desk and exhaust the air from the return tubes. Both ends of the tubes are then left open at the counters, no sending or receiving apparatus being required there. The carriers are so light, their velocity so low, and the air-pressure varies so little from the pressure of the atmosphere that the carriers can be allowed to drop out of the tubes on to the counters, and they can be despatched by simply placing them at the open end of the tube into which the air is flowing. The currents of air entering and leaving the tubes are not so strong as to cause any special annoyance. At the cashier’s desk some simple receiving and sending apparatus has to be used, but it is better to concentrate all of the apparatus at one point rather than have it distributed about the store, as would be the case if the double system were not used. In the London pneumatic telegraph both methods of operation have been in use. Double lines were laid, and the engines exhausted the air from one tube and forced it into the other. The exhausted tube was therefore used to despatch in one direction, while the other tube, operated by the compressed air, served for despatching in the opposite direction. So far as I know, both work equally well.