THE THERMOMETER. 



137 



the mercury, when the thermometer is immersed in boiling water, will always 

 stand at the same point. This, then, is another fixed point of temperature, 

 which may be determined at all times, and in all places, and is called the 

 boiling point. Let the point at which the column of mercury stands, under 

 these circumstances, be marked on the scale. 



The interval between the freezing and boiling points, thus ascertained, is the 

 portion of the tube which corresponds to the expansion of the mercury between 

 these two points of temperature, and this expansion is necessarily always the 

 same ; consequently the proportion which the capacity of the tube between these 

 two points bears to the volume of mercury contained in it at the temperature 

 of melting ice must always be the same. If a number of different thermome- 

 ters, prepared in a manner similar to that already described, be submitted to 

 this process, it will be found that the intervals between the freezing and boiling 

 points in them, severally, will differ in length. The capacities of the tubes, 

 between these points, however, will always bear the same proportions to the 

 capacities of those parts of the instrument below the freezing point, including 

 the bulb. This is a necessary consequence of the uniform expansion of mer- 

 cury when submitted to the same limits of temperature. It is ascertained that 

 between the boiling and freezing points, the expansion of the mercury amounts 

 to one sixty-third part of its bulk, at the temperature of melting ice ; conse- 

 quently the capacity of the tube between the temperature of melting ice and 

 boiling water, must always be equal to one sixty-third part of the capacity of 

 the bulb, and that part of the tube below the mark indicating the temperature 

 of melting ice. The different lengths of the intervals in different thermometers 

 between the freezing and boiling points will, therefore, arise from the different 

 proportions which the capacity of that part of the tube bears to the capacity of 

 the bulb, and the portion of the tube below the mark indicating the freezing 

 point. 



Thermometer thus constructed would, at all times and places, determine the 

 temperatures of all bodies whatsoever, whose temperatures were equal to those 

 particular ones which have been marked on the scale. 



Instruments thus constructed would determine with certainty whether the 

 temperature of bodies to which they were exposed were greater or less than 

 those of melting ice or boiling water ; but could two philosophers, instituting 

 experiments in different countries corresponding with each other, declare the 

 exact quantity by which the temperature of any body to which the thermometer 

 was exposed exceeded or fell short of those fixed temperatures ? To do so, he 

 would naturally inquire by what proportion of the whole interval between the 

 freezing and boiling points the column stood above or below either of these 

 fixed terms. Thus, if he were able to declare that the column stood at a point 

 between the fixed terms at a distance above the freezing point equal to one 

 third of the whole distance between the freezing and boiling points, he would 

 enable another philosopher, in a distant country, to repeat the same experiment, 

 and to compare the results. In* order, therefore, perfectly to estimate these 

 proportional distances, the scale attached to the thermometer is further divided, 

 and the interval between the temperatures of melting ice and of boiling water 

 is divided into a number of equal parts previously agreed upon ; and that being 

 done, the same divisions are continued above the term of boiling water and 

 below the term of melting ice. The number of divisions into which the inter- 

 val between the fixed points of temperature is divided, being altogether arbitrary, 

 has been differently determined in different countries, and by the different con- 

 trivers of thermometers. The thermometer commonly used in this country, 

 and called Fahrenheit's thermometer, has its interval divided into 180 equal parts, 

 called degrees ; and these divisions are continued upward and downward. 



