According to the division generally used in this country, the space between the freezing and boiling points consists of 180 degrees. Fahrenheit, who first employed this scale, and whose name it still bears, imagined that the greatest cold that could be produced, is that which results from a mixture of snow and sea salt, and he adopted that temperature as the com mencement of the scale, which he accordingly marked zero, or o. He then exposed the instrument to the heat of boiling mercury, and having marked the point to which it rose, he divided the space between that point and the ,lower extremity into 600 equal parts. He afterwards found that the melting point of snow corresponded with the 32d division, and the boiling point of water with the 2I2th. Hence, in what are called Fahrenheit's thermometers, the freezing point is 32, and the boiling point 212, the difference between them being 180. The mode of graduating the scale originally adopted by Fahrenheit, has been long ago abandoned for that stated above ; but the graduation itself, though liable to many objections, is still retained, and it is to this scale that all our references in this ar ticle are made. This graduation is represented on the side CD of the scale, in Fig. 1.
Among the various thermometrical scales that have at different times been proposed, that which is now generally used in France, and known by the name of the centigrade, is perhaps upon the whole the most eli gible. The distance between the freezing and boiling points is divided into 100 equal parts, the freezing point being marked 0, and the boiling point 100. We must refer to our article THERMOMETER, for a particu lar account of the methods by which the indications of different thermometers may be compared ; but we may here remark, that when any given temperature is ex pressed in degrees of the centigrade, it may be con verted into these of Fahrenheit, by the following simple rule. Multiply the degrees of the centigrade by 9, di vide the product by 5, and add 32 to the quotient. Thus when the centigrade stands at 25, Fahrenheit, in the same circumstances, will indicate 77, for 25 multiplied by 9, and divided by 5, gives 45, which added to 32 is 77. The divisions in every thermometer are numbered from zero downwards, in the same way as from zero upwards ; but to distinguish them, the former are written with the negative sign or minus prefixed. Thus —15 means a temperature 15 degrees below 0. The degrees, accord ing to this graduation, are represented on the side EF of the scale, in Fig. 1.
Thermometers are sometimes constructed of alcohol, or spirit of wine, which, though inferior in many re spects to mercury, is on certain occasions preferable, and indeed necessary, as for example, in ascertaining temperatures below the point at which mercury freezes.
In an alcohol thermometer, the freezing point may be ascertained by plunging it into melting snow, as stated above, but the other divisions must be determined by comparing it with a mercurial one, previously graduat ed as alcohol passes into a state of vapour long before it arrives at the heat of boiling water. Even tnereu vial thermometers are frequently graduated in this way, the tube in many of them being too short to admit of the instrument being exposed to so elevated a tempera ture.
In meteorological observations, it has been found very desirable to have some method of ascertaining the greatest degree of heat and cold, daring any given in terval, in the absence of the observer. Various con trivances have accordingly been employed for accom plishing this, by means of what are called self-regis tering thermometers. Those commonly in use are of a very simple construction. A mercurial thermometer, AB, (Plate CCCLXXIV. Fig. 2.) of a somewhat wider bore than ordinary, with a small bit of steel wire a over the mercury, so as to slide easily up and down in the tube, is placed in a horizontal position. As the tempera ture increases, the mercury pushes forward the bit of steel or index ; but when the mercury again retires, in consequence of a diminution of temperature, the index remains behind at the highest point to which the mer cury has risen. A thermometer AB (Fig. 3.) of a similar bore, but filled with spirit of wine, and having a small thread of glass a, about half an inch in length, immersed in the fluid, is also placed in a horizontal position. As the temperature diminishes, and the spirit sinks in the tube, it is found that the surface of the liquid does not pass the glass index, but carries it along with it, though, when an increase of temperature again takes place, and the spirit rises in the tube, the index is left behind at the lowest point to which the liquid had sunk. To prepare the instruments for a new observa tion, they are both inclined, so as to bring the index in each to the surface of the liquid, when they are again placed in a horizontal position. In referring to these instruments in the course of this article, we shall deno minate the first a maximum, and the last a minimum thermometer. For a more particular account of the method of constructing these, and others of the same kind, see the article 'THERMOMETER.