344 HENEY A. KOWLAND 



is used almost daily by the physicist. Yet, when we come to consider 

 the history of the subject carefully, we find that the only experimenter 

 who has made the determination with anything like the accuracy 

 demanded by modern science, and by a method capable of giving good 

 results, is Joule, whose determination of thirty years ago, confirmed 

 by some recent results, to-day stands almost, if not quite, alone among 

 accurate results on the subject. 



But Joule experimented on water of one temperature only, and did 

 not reduce his results to the air thermometer; so that we are still left 

 in doubt, even to the extent of one per cent, as to the value of the 

 equivalent on the air thermometer. 



The reduction of the mercurial to the air thermometer, and thence 

 to the absolute scale, has generally been neglected between and 100 

 by most physicists, though it is known that they differ several tenths 

 of a degree at the 45 point. In calorimetric researches this may pro- 

 duce an error of over one, and even approaching two per cent, especially 

 when a Geissler thermometer is used, which is the worst in this respect 

 of any that I have experimented on; and small intervals on the mer- 

 curial thermometers differ among themselves more than one per cent 

 from the difference of the glass used in them. 



Again, as water is necessarily the liquid used in calorimeters, its 

 variation of specific heat with the temperature is a very important 

 factor in the determination of the equivalent. Strange as it may 

 appear, we may be said to know almost nothing about the variation 

 of the specific heat of water with the temperature between and 

 100 C. 



Regnault experimented only above 100 C. The experiments of 

 Hirn, and of Jamin and Amaury, are absurd, from the amount of varia- 

 tion which they give. Pfaundler and Platter confined themselves to 

 points between and 13. Miinchausen seems to have made the best 

 experiments, but they must be rejected because he did not reduce to 

 the air thermometer. 



In the present series of researches, I have sought, first, a method 

 of measuring temperatures on the perfect gas thermometer with an 

 accuracy scarcely hitherto attempted, and to this end have made an 

 extended study of the deviation of ordinary thermometers from the 

 air thermometer; and, secondly, I have sought a method of determin- 

 ing the mechanical equivalent of heat so accurate, and of so extended 

 a range, that the variation of the specific heat of water should follow 

 from the experiments alone. 



