Dear Sir, Your most obedient humble Servant, And. Mitchell. _P.S._ I may add, that the distance from mount Hecla to Zetland is between 500 and 600 miles. XXXVIII. _A Description of some Thermometers for particular Uses. By the Right Honourable the Lord_ Charles Cavendish, _V.P.R.S._ [Read June 30, 1757.] THE thermometer (TAB. XI. _fig._ 1.) is designed for shewing the greatest degree of heat, which happens in any place during the absence of the observer. It consists of a cylinder of glass joined to a tube, and differs from common thermometers only in having the top of the stem drawn out into a capillary tube, which enters into a glass ball C, joined on to the stem at the place where it begins to be contracted. The cylinder, and part of the tube, are filled with mercury; the top of which shews the common degrees of heat as usual. The upper part of the tube above the mercury is filled with spirit of wine, and some of the same liquor is left in the ball C, so as to fill it almost up to the top of the capillary tube. Now when the thermometer rises, the spirit of wine will be driven out of the tube, and will fall into the ball C. When the thermometer sinks again, as the spirit cannot return back from the ball, the top of the tube will remain empty, and the length of the empty part will be proportional to the fall of the thermometer. Therefore, by means of a proper scale, the top of the spirit of wine will shew how many degrees it has been higher than when observed; which being added to the present height, will give the greatest degree of heat it has been at. [Illustration: _Philos. Trans. Vol. L._ TAB. XI. _p. 300_. _J. Mynde sc._] To fit this thermometer for a new observation, it is necessary to fill the upper part of the tube with spirits; which may be done, by inclining the instrument till the spirits in the ball C cover the end of the capillary tube. For if the cylinder is then heated, by applying the hand to it, or by the flame of a lamp held at some distance, till the spirits rise to the top of the tube and run over into the ball C, and is then suffered to cool in the same position, the tube will remain full of spirits, and the thermometer will be fitted for a new experiment. The top of the capillary tube is made to stand pretty near to one side of the ball, and also to the top of it, that a less inclination of the instrument may be sufficient to make the spirit of wine in the ball cover the end of the tube. The ball C is joined on as high as possible, so as to hide no part of the tube, except that, where the bore is contracted. By this means, the top of the spirit of wine begins to appear before the thermometer has sunk one degree.