It is not an easy matter for an inexperienced person to fill a barometer properly. The tube and mercury must first be made warm. The mercury may be heated to the boiling point of water in an iron vessel; a vessel having tin in its composition must on no account be used. The glass should be warmed sufficiently to ensure the evaporation of all moisture. Make a paper funnel having but a very small aperture and pour in the mercury, whose impurities will cling to the paper funnel, and test for correct amount with a standard barometer. Be careful that air does not enter with the mercury. If an odd air-bubble appears, send up a little more to collect, and send up to the top what has already entered.