It will be better to leave a little of the spirit above the mercury in the longest leg; in which case the top of the spirit will shew the common degrees of heat. For the filling the tube, so as to leave none, is attended with some trouble; and more of it will be apt to get up there, if the instrument should happen to be held in an improper situation, or if it be kept in too warm a place without filling the shorter leg with mercury by the method above described. If too great a quantity should get up, tho’ it would not affect the scale for the common degrees of heat, it would however cause some error in the degrees on the shorter leg; inasmuch as the expansion of that portion of spirits, which has got up into the longer leg, exceeds the expansion of the mercury, which must supply its place. It may be got back at pleasure, by exposing the thermometer to such a degree of cold as will make the spirit get beyond the bend of the syphon; for then it will run up along the thread of glass in the shorter leg till it gets above the mercury there. For this purpose the point of 0 degrees of Fahrenheit’s scale should be near the bend; by which means, any part of the spirit of wine may be got beyond it by an artificial cold; and there will be no danger of the whole getting beyond it by any natural cold; in which case the air would get up into the ball. The scale of degrees on the shorter leg will, in different seasons, be liable to an error of the same kind as that, which was explained in the first-mentioned thermometer; but in this it will be less considerable, as the space between the two scales is filled with mercury, whose expansion is about six times less than that of spirit of wine. In the thermometer, which I have, the bore of the tube is about 0.054 inches; and one inch of it contains eight grains of mercury, and answers to seven degrees of Fahrenheit’s scale. The drops of mercury, which fall into the ball A, answer to about one eighth of a degree. If instruments of the nature of those above described, were to be used for finding the temper of the sea at great depths, some alteration would be necessary in the construction of them, principally upon account of the great pressure of the water; the ill effect of which can, I believe, be prevented no other way, than by leaving the tube open. For if the thermometer was made strong enough to resist the pressure without breaking, yet it would be impossible to be sure, that the figure of the glass would not thereby be altered, which should make the experiment uncertain. The instrument for finding the greatest heat might be made just like that of fig. 1. only leaving the top open. It is to be filled with mercury only, as is also the lower part of the ball at top, but not near so high as the end of the capillary tube. The upper part of that ball, being left open, will in a great measure be filled with the sea-water, which will be forced into it by the pressure.