We worry because we are able through a power called self-consciousness to place ourselves through our minds for the time being. Either--back somewhere in the past without carrying our physical bodies with us; for if we could take our bodies with us, we would be in the present again, and then worry is impossible; or, we use our imagination and project the future entirely apart from our bodies, for we cannot project our bodies into the future, and if we could we would again be in the present. We worry over going to have an operation performed which may or not be dangerous, but quite necessary. We may still think we worry when the operation begins, but as soon as that occurs the time becomes the present, and though we may fear, we cannot worry in the present. [Illustration: _Back View of Shield_ _Longitudinal Section through Shield & Tunnel_ _Diagram showing method of tunnel construction by shield and compressed air._ _Scale; ¹⁄₈ inch · 1 foot_ _Jacobs & Davies Inc. 30 Church St. N.Y._ _Oct. 15. 1910._ FIGURE 1.] The Story in a Tunnel How a Tunnel Is Dug Under Water. Fig. 1. On the left is a cross section showing, in diagram, the back view of a shield. The heavy black circle is the “tail” or “skin.” The small circles within the tail are the hydraulic rams which at a pressure of 5,000 pounds to the square inch force the shield forward. The square compartments within the shield are the openings through which the men pass to dig away the ground. In the middle of the shield is shown the swinging “erector” which picks up the iron lining plates and puts them in position. The view on the right is a longitudinal section of the tunnel showing the shield and the bulkhead wall across the tunnel with the air locks built into it. The front of the shield ahead of the doors is made with a sharp edge called the “cutting edge” and this makes it easier for the shield to advance in case all the ground in front has not been removed. This view shows how the tail overlaps the last portion of the iron lining. Some distance behind the shield comes the concrete bulkhead wall with the air locks contained in it. There are two shown in the view. The upper one is the emergency air lock, always kept ready so that in case of an accident the men have a means of escape even though the lower part of the tunnel is filled with rushing water or mud. The lower air lock is for the passage of men and materials during ordinary working. This view also shows that all the tunnel ahead of the bulkhead wall is under compressed air while the finished tunnel behind the bulkhead wall is under the ordinary or normal air pressure. When the tunnel is finished the air locks and bulkhead walls are removed. [Illustration: FRONT VIEW OF A DRIVING SHIELD