216 THE POPULAR SCIENCE MONTHLY. 



about this period, 1 that the heat so generated " is exactly proportional 

 to the force with which the tico surfaces are pressed together, and to 

 the rapidity of the friction:'''' in other words, that the production of 

 heat is "exactly proportional" to the work expended in producing it. 

 First drawing attention to the absurdity of an apparatus contain- 

 ing or creating an indefinite supply of a material substance; then 

 proving by experiment that the quantities of heat excited in a given 

 time were proportional to the expenditures of an entirely different 

 magnitude work: he must be credited not only with the first 

 conclusive, but with the most weighty argument initially available, 

 against the existence of caloric, or in favor of the dynamic origin of 

 heat. 



-+*- 



OPEN" AIR AND HEALTH. 2 



By Dr. PAUL NIEMEYER. 



I SPARE the reader the diffuseness of an introduction, by telling 

 him of a scene in an omnibus, which hinged on the question wheth- 

 er the conductor should open or shut the windows. On the left was 



in which the substitution of Count Rumford's data, 



p 10,000 lbs., a 0.3 inch, r = 1.75 inches, 



gives for the approximate moment of friction of the borer, in foot-pounds, 



800/ 



So that, making thirty-two revolutions per minute, a quantity of work, 160,800/, would 

 be expended during the same interval. 



On the other hand, the heat excited in two hours and thirty minutes, and which, dy- 

 namically, was to be regarded the equivalent of the work expended, according to Count 

 Rumford's estimate, was sufficient to raise the temperature of 26.58 pounds of water 180 

 Fahr., or 4,784 heat-units. The production of one heat-unit, therefore, corresponded to 

 the expenditure approximately of an amount of work 



5041/ 

 For /= 0.15, this would give 756 



" /= 0.20, " " " 1008 



as the equivalents in mechanical units or foot-pounds of one British thermal unit. 



Prof. Tait, availing himself of the remark let fall by Rumford, that " the machinery 

 used in this experiment could easily be carried round by the force of one horse," and as- 

 suming 30,000 foot-pounds as the value of a horse-power per minute, thus derives 

 940 foot-pounds as the mechanical value of a rise of temperature of 1 Fahr. in one 

 pound of water. (See " Historical Sketch," p. 9.) But Prof. Thurston regards this cal- 

 culation as unfair to Rumford, quoting Rankine's estimate of the admissible value of a 

 horse-power, 25,920, from which the value of the equivalent, 812, results. This critique 

 also seems the more allowable, since Rumford neither made corrections for the work ex- 

 pended in friction in " the complicated machinery used " in the determination, nor for 

 " the heat accumulated in the wooden box, nor for that dispersed during the experiment." 

 (See Journal of the Franklin Institute, 3, lxvii., p. 203.) 



1 " Kleine Schriften," 1805, vol. iv., p. 41. "Complete Works," Am. Ac. ed., vol. 

 ii., p. 209. 



2 Translated from the German, by J. Fitzgerald, A. M. 



