It will be convenient to leave some mercury in the ball C, which may be made to cover the end of the capillary tube, by inclining the thermometer more than what is necessary to make the spirit of wine cover it. By this means some mercury may be got back into the tube, in case any of it should happen to be driven into the ball by the thermometer’s being exposed to too great a heat. The scale of degrees at top, which shews the descent of the thermometer from the highest point it has arrived at, ought not, in strictness, to be the same at all times of the year; for those degrees exceed the common degrees of heat pointed out by the top of the mercury, as much as the column of spirit of wine expands, and therefore are greatest when that column is so; that is to say, when the greatest heat to which the instrument has been exposed is least. A difference of 30 degrees of Fahrenheit’s scale, in the greatest rise of the thermometer, would require the scale to be altered one sixtieth part: and the error arising from making use of the same scale will be about one sixth of a degree, if the thermometer is observed when it has fallen ten degrees. In the instrument here described, the bore of the tube is about 0.027 inches; and one inch of it contains two grains of mercury, and answers to about ten degrees, the cylinder containing about 2280 grains. If a much shorter tube was made use of, a considerable error might arise from too great a quantity of spirits adhering to the sides of the tube, in that part, which is filled with mercury; especially when the thermometer rises fast. This makes it necessary to employ a cylinder of a considerable bigness, if it is desired to have the scale of degrees pretty large. If the weight of the mercury is thought inconvenient, it may be avoided by the construction described in fig. 2. where the bottom of the tube is bent so as to point upwards, and is joined to a ball A, which communicates with a cylinder placed above it. In all other respects it is the same as the instrument before described. It is filled with spirit of wine and mercury; the quantity of the latter being sufficient to fill the whole tube and the ball A. No part of the spirit, with which the cylinder is filled, can get into the tube, as long as the instrument is kept in an erect position, or even if it is carefully laid down flat on a table. For tho’ in this last case some of the spirits may get into the ball A, it will rise to that part of the ball, which is then uppermost, and will not touch the orifice of the tube _n_; which was the reason for adding this ball, which would be unnecessary, if the instrument was kept constantly erect, or nearly so. If the spirit should come to touch the orifice of the tube _n_, it would work up between the mercury and the glass; which would put the instrument out of order.