The Mcleod Guage consist of a bulb and capillary tube where their volume of mercury v1 is at atmospheric pressure P 1. When vacuum is applied both are evacuated and when the guage is rotated, this volume is compressed to a small portion in the capillary having volume V2 at corresponding pressure (i.e. vacuum )P2
Boyle's Law P1V1 =P2V2

The Pressure is read by comparing the compressed level of mercury with the corresponding graduated scale parallel to the capillary tube.