III. On the construction of a glycerine barometer

Abstract

Various attempts have been made from time to time to construct barometers with fluids of lower density than mercury, with the view of increasing the range of movement of the column by the direct action of the atmosphere, thereby rendering the variations of pressure more easily visible, without the intervention of mechanical appliances. I have been induced to give the subject attention in the belief that if precise instruments of this class could be made they would prove of scientific value in showing the character of the more minute vibrations of atmospheric pressure, and of practical use at storm stations, collieries, and other situations where it is of importance for the unpractised eye to notice frequently the movements of the column without the careful observation necessary in the reading of an ordinary mercurial barometer. Water is apparently the most convenient liquid for a long range barometric column, and many water barometers have been constructed, notably that made for the Royal Society in 1830, by Professor Daniell. A water column is, however, of little use for indicating variations of pressure, owing to the effect of changes of temperature on the water vapour existing in the Torricellian vacuum, which often masks the effects which would otherwise be produced by changes of pressure. Having successfully constructed several water barometers, and finding this conflicting action caused by variations of temperature so detrimental to their value, I was induced to experiment with other liquids, and among those tried pure glycerine appeared to me to answer the purpose best. Glycerine from its high boiling point has a very low tension of vapour at the ordinary temperatures of the atmosphere; the length of the column is therefore only altered from changes of temperature by expansion and contraction of the liquid itself, and that in a very small degree, the mean coefficient of absolute expansion between 0° and 100°C. being ·0005455. The specific gravity of the purest glycerine as manufactured by Messrs. Price and Co. is 1·26, or less than one-tenth that of mercury; the mean height of the column is 27 feet at sea level, and a variation of a tenth of an inch in the height of the mercurial column is shown by a change of more than an inch in the glycerine; the boiling point is 440° F., and a very low temperature is required to solidify it. As glycerine absorbs moisture freely if exposed to the atmosphere, the action is prevented by covering the surface of the liquid in the cistern with a shallow layer of heavy petroleum oil, prepared especially for the purpose.