Enquire within upon everything by Robert Kemp Philp

977. _Why does Tapping the Face of the Barometer sometimes cause the

Hand to Move?_ Because the weight on the surface of the mercury frequently leans against the side of the tube, and does not move freely. And, also, the mercury clings to the sides of the tube by capillary attraction; therefore, tapping on the face of the barometer sets the weight free, and overcomes the attraction which impedes the rise or fall of the mercury.