The ornamental design for a thermometer, as shown and described.
FIG. 1 is a front and top perspective view of a thermometer, showing my design.
FIG. 2 is a front elevation view thereof.
FIG. 3 is a rear elevation view thereof.
FIG. 4 is a left side elevation thereof.
FIG. 5 is a right side elevation thereof.
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines shown in the drawings are included for the purpose of illustrating portions of the thermometer that form no part of the claimed design.