The ornamental design for a continuousspring, as shown and described.
FIG. 1 is a perspective view of a continuous spring illustrating our new design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a front view thereof; the rear view being identical; and,
FIG. 5 is a right side view thereof, the left side being identical.
The continuous spring is shown with symbolic drafting cut lines at the ends to indicate that this is a section of a continuous spring. The cut lines do not form part of the claimed design.