ABCD, fig. 9. is a smooth flat plate; and EF and GH two rulers or flat pieces, a quarter of an inch thick, fixed flat upon the plate, with the sides that are towards one another made perfectly true, a little farther asunder at one end EG than at the other end FH: thus they include between them a long converging canal, which is divided on one side into a number of small equal parts, and which may be considered as performing the offices both of the tube and scale of the common thermometer. It is obvious, that if a body, so adjusted as to fit exactly at the wider end of this canal, be afterwards diminished in its bulk by fire, as the thermometer pieces are, it will then pass further in the canal, and more and more so according as the diminution is greater; and conversely, that if a body, so adjusted as to pass on to the narrow end, be afterwards expanded by fire, as is the case with metals, and applied in that expanded state to the scale, it will not pass so far; and that the divisions on the side will be the measures of the expansions of the one, as of the contractions of the other, reckoning in both cases from that point to which the body was adjusted at first.
I is the body whose alteration of bulk is thus to be measured. This is to be gently pushed or slid along towards the end FH, till it is stopped by the converging sides of the canal. See CHEMISTRY, N° 1412.
A very ingenious application of Fahrenheit's thermometer has been made by Mrs Lovi, glass-blower in Edinburgh, for ascertaining the temperature of compost dunghills, for regulating the temperature of hot-beds, and observing the changes of temperature in corn and haystacks when they are put up damp. This may be called an agricultural thermometer, and has been found of great use for the above purposes.