THE BEGINNINGS OF LIFE. 1 1 



speculate on these subjects. He noticed that the 

 venous blood of the tropics was of a much brighter red 

 than in colder latitudes, and his reasoning on this fact 

 led him into the laboratory of natural forces, where he 

 has worked with such signal ability and success.' But 

 in the following year, 1843, Mr. Joule of Manchester 

 published his first paper on the c Mechanical Value of 

 Heat,' in which he detailed the most valuable results 

 of a series of experiments, conducted whilst he was 

 in ignorance of the labours of Seguin and of the reason- 

 ings of Mayer. It is to him that we are principally 

 indebted for the actual experimental determination of 

 the mechanical equivalent of heat. A paddle-wheel 

 was made to revolve in a copper vessel containing 

 a weighed quantity of water at a known temperature. 

 The mechanical force, derived from falling weights, 

 which was employed in turning the wheel was known ; 

 so that when, after the wheel had revolved for a cer- 

 tain time, the temperature of the water was estimated, 

 and the distance through which the weights had fallen 

 in the same time was computed, it became easy to 

 estimate the quantity of heat which corresponded to 

 the fall of a known weight through a given distance. 

 Of course, corrections had to be made, allowing for 

 the heating of the copper vessel, and of the wheel itself, 

 as well as for the loss of heat by radiation. Similar 

 experiments were conducted with oil and with mer- 

 cury, though under somewhat different conditions ; and 

 in all cases the amount of heat evolved by the friction 



