The endless experiments and tests made within the last fifteen years by those who have been connected with the practical rather than the theoretical end of the question of ventilation tend to show that a single fan system of ventilation will not accomplish all that can be done. with a double system. In order to make myself thoroughly understood, let me ask what we are trying to get rid of where we employ mechanical means of ventilation? Is it not the old "natural" or "gravity" method? Again, I ask what is it we are trying to get rid of most? Is it not the heavy and foul air in the room to be ventilated?

I think we can all agree that mechanical systems are designed to take the place of the unreliable and variable gravity methods. I think we can also agree that it is the foul air in the rooms that we wish to rid ourselves of when we adopt methods of ventilation. If I am correct in assuming that we can agree on these two points, then we are down to a basis where we can discuss reasonable means of accomplishing the best results. We will agree most cheerfully that it is all important that we furnish plenty of fresh air to the occupants in any room that we wish to ventilate. You cannot take water out of a barrel unless you first put water into it; neither can you take air out of a room unless you provide for a proper air intake. In the old gravity methods the air was

taken into the building by what may be well termed "coaxing" methodsthat is, the presence in the basement of a heat chamber produced a tendency to a vacuum, which caused the air to flow from the outside towards that vacuum and, being heated, to rise through flues into the room or rooms to be heated and ventilated. The flow was assisted by taking advantage of the air currents themselves. Naturally, such a system depended almost wholly upon atmospheric conditions, and was equally uncertain and variable. To make sure of this flow being the same at all times the "supply blower" is now used with admirable results. There is no question that such a system wholly overcomes the variable and uncertain flow of gravity means.

The funny part to me is that it is seriously contended that what is good for supplying constantly the requisite quantity of pure air is wholly unnecessary for the exhaust of the foul air-the very thing you want to get rid of most in any system of ventilation. Such a contention, I know, is mostly based on the supposition that the pressure exerted to force the air into the room will force it out. On that question I wish to quote from the report of the late Chief Wade, of the Massachusetts District Police, to show what has been demonstrated in actual practice:

"The forcing of abundant fresh air into a room will drive the air out