The next application on our list, pressure tank pumps in connection with elevators, represents a somewhat smaller percentage of H. P. hours of actual service in the motor as compared with the possible H. P. hours than in the case of an open tank pump. In case of the pressure tank the water reserve is usually limited, and the motor therefore must be equal to the continuous operation of the elevator at maximum load. Taking this fact into consideration, and the circumstances of elevator use being about the same in this case as in the case of the open tank elevator, we have a greater ratio of difference between the possible or contract H. P. hours in the motor and the H. P. hours of actual service, the maximum average use being about 70 per cent. to 75 per cent. and the minimum as low as 35 per cent. to 40 per cent., depending, of course, on the character of building in which the elevator is employed or the character of service. In calculating the size of motor required on an elevator of this description, a very convenient fact to remember is that every pound of pressure per square inch is equivalent to lifting water about 23 ft., or about 230 ft. per 100 pounds pressure, By reducing the required pressure to a relative lift in feet, and knowing the amount of water required by the elevator per minute, the motor calculation becomes the same as in case of the open tank elevator, the same allowances being made for friction, etc., as in the first case. The regulation of the motor in this case should be accomplished by the conditions of pressure in the pressure tank, as is the case with a steam pump employed in this service.