8990000 Where h denotes the height of a thermo meter plunged in boiling water above the point or melting ice, in degrees of Bird's Fahrenheit, and z the height of the barometer in 10ths of an inch. From this rule he has computed the following table, for finding the heights to which a good third's Fahrenheit will rise, when plunged in boiling water, in all states of the barometer, from 27 to 31 English in dies ; which will serve, among other uses, to directinstrtunentmakers m mak ing a true allowance for the effect of the variation of the barometer, if they should be obliged to finish a thermometer at a time when the barometer is above or be low 30 inches ; though it is hest to fix the boiling point when the barometer is at that height.
The numbers in the first column of this table express heights of the quicksilver in the barometer, in English inches and decimal parts : the second column shows the equation to be applied, according to the sign prefixed, to 212° of Bird's renheit, to find the true boiling point for every such state of the barometer. The boiling point, for all intermediate states of the barometer, may be had, with suffi cient accuracy, by taking proportional parts, by means of the third column of differences of the equations.
The method of constructing Fahren heit's thermometer, which is now in gene ral use in this country, is the following:' a small ball is blown on the end of a glass tube, of an uniform width throughout. The ball and part of the tube are then to be filled with quicksilver, which has been previously boiled to expel the air. The open end of the tube is then to be her metically sealed. ' The next object is to construct the scale. It is found, by periment, that melting snow, or freezing water, is always at the same temperature. If, therefore, a thermometer be immers ed in the one or the other, the quick silver will always stand at the same point. It has been observed, too, that water boils under the same pressure of the at mosphere at the same temperature. A thermometer, therefore, immersed in boiling water, will uniformly stand at the same point. Here, then, are two fixed points, from which a scale may be con structed, by dividing the intermediate space into equal parts, and carrying the same divisions as far above and below the two fixed points as may be wanted. Thus, thermometers constructed in this way may be compared together ; for if they are accurately made, and placed in the same temperature, they will always point to the same degree on the scale. The
fluid, as we have seen, employed is quick silver, and it is found to answer best, be cause its expansions are most equable. The freezing point of Fahrenheit's ther mometer is marked 32° ; and the reason of this is said to have been, that this artist thought that he had produced the great est degree of cold, by a mixture of snow and salt ; and the point at which the ther mometer theirstood, in this temperature, was marked Zero. The boiling point, in this thermometer, is 212°, and the inter mediate space, between the boiling and freezing points, is therefore divided into 180°. This is the thermometer that is commonly used in Britain.
There are three other thermometers employed in different countries of Eu rope, which differ from each other in the number of degrees between the freezing and boiling points. Reauniur's thermo meter was generally used in France be fbre the revolution, and is still employed in different countries on the Continent. The freezing point, in this thermometer, is marked Zero, and the boiling point ay. To convert the degrees of Reau mur's thermometer to those of Fahren heit, the following is the formula. Rearm. x 9 32 = Fain% that is, multiply the degrees of Reaumur by 9, divide by 4, and add 32. This gives the correspond ing degrees on Fahrenheit's scale. The thermometer of Celsus has the space be tween the freezing and boiling points di vided into 100°. The boiling point is and the freezing point Zero. This thermometer is" used in The " thermometre centigrade," now used in France, has the scale divided in the same way. To convert. the degrees of this ther mometer into those of Fahrenheit ; Cel.
X 9 32 = Fahr. In Delisle's thermo 5 meter, which is used in Russia, the space between the boiling and freezing points is divided into 15Q°; gut the degrees are reckoned downwards. The boiling point is marked Zero, and the freezing point 150°. To reduce the degrees of this ther mometer under the boiling point to those of Fahrenheit ; Del. 212 = Fahr.
X 6 And above the boiling point. Del. + 212 = Fahr.
• Such, then, are the principles and mode of construction of the thermometer ; an instrument which has been of the utmost importance in enabling us to discover many of the properties and effects of ca loric, as by it only we can ascertain, with accuracy, the relative temperatures.