viii PREFACE. 



use of his scale, in consequence of which all tabular quantities and results pre- 

 sented in this paper have reference to this graduation. For the sake of uniformity, 

 records originally given in Reaumur or Centigrade scale have been converted into 

 that of Fahrenheit, and however advisable otherwise it might have been to adopt 

 the Centigrade scale, such a step was forbidden by the great labor and consequent 

 expense which the conversion would have entailed. 



on proper principles, thermometers upon which reliance could be placed ; his earlier instruments were 

 filled with alcohol, but about the year Itli he used mercury for this purpose. According to his 

 own account, he recognized three principal points, viz. : his so-called absolute zero, representing the 

 extreme cold experienced by him in the severe winter of 1709 and erroneously supposed to indicate 

 the greatest cold, the freezing point of water, and a point representing the heat of the human body ; 

 in practice, however, he made use of the freezing point as well as of the boiling point of water, with 

 the fixity of which latter he became acquainted in 1714. Supposing the volume of mercury at the 

 temperature represented by his zero point to be 11124 parts, he noticed an expansion of 32 parts at 

 the temperature of freezing water, and of 212 parts at the temperature of boiling water, and accord- 

 ingly adopted the numbers 32 and 212 to indicate these temperatures. Before Fahrenheit's instru- 

 ments came into general use, Reaumur brought out his spirit thermometers graduated between the 

 freezing and boiling points of water from to 80, and shortly after, Celsius, about 1742, introduced 

 the Centigrade division between the same points. The spirit thermometers used in the preceding 

 century had arbitrary scales, and were not generally directly comparable. * * * Fahren- 

 heit had already noticed the effect of a change in the atmospheric pressure on the position of the 

 boiling point, but the proper allowance or reduction to a standard pressure was not satisfactorily 

 ascertained in his time. It would seem that allowance was made for the expansion of the glass 

 tube in the above-mentioned experiment, since the dilatation of mercury is nearly 0.0001 of its 

 volume for 1° Fah. All of the thermometric scales mentioned are intended to measure equal incre- 

 ments of heat by equal increments in their scale readings, but for the purpose of comparison and 

 discussion it is much to be desired that all should agree to use the same scale, the Centigrade scale 

 being the one most likely to take the place of the others. 



In connection with the cold indicated by the zero of Fahrenheit's scale it may be remarked as an 

 accidental circumstance, that it may and has been taken roughly to be that of the mean annual 

 temperature of the pole, hence the possibility of representing approximately the annual mean 

 temperature in the latitude 4> by the simple expression 81°.5 cos $ wilhoul the addition of a constant. 



