Cyrus Harding1, dipping in his hand, felt the water oily to the touch. He tasted it and found it rather sweet. As to its temperature, that he estimated at ninety-five degrees Fahrenheit2. Herbert having asked on what he based this calculation,―
"Its [sic]3 quite simple, my boy," said he, "for, in plunging my hand into the water, I felt no sensation either of heat or cold. Therefore it has the same temperature as the human body, which is about ninety-five degrees."
Jules Verne, The Mysterious Island
The National Weather Service is predicting that today the air temperature will go up to about 36° C (96° F) around Washington, DC. So, by Jules Verne's logic, it should feel just right and quite comfortable outside. Which is, of course, total nonsense.
Thermodynamically, no chemical process can be 100% efficient. As a result, all metabolic activities produce heat, which is what humans and all other animals use to warm up their bodies. However, because humans cannot turn off their metabolism, heat is always generated and the excess must be dissipated to the surroundings to prevent the body from overheating above the normal temperature of about 36.7° C.
Whenever a body is at a higher temperature than its surroundings, the rate at which energy is lost to the surroundings in the form of heat is proportional to the temperature difference between the body and the surroundings. In other words, as the temperature of the body approaches that of the surroundings, less and less heat is transferred to the surroundings. And that is why when the ambient temperature is near the normal body temperature, rather than feel comfortable, we feel hot, because our bodies begin to overheat.
The situation is slightly different in water, because water is more efficient than air in conducting heat. That is why being in water at 36° C feels more comfortable than being in air at 36° C. Nevertheless, when you put your hand in water at 36° C it feels warm. Believe me, because I tried it last night. I filled up a sink with water while adjusting the temperature to about 36° C by turning the hot and cold water faucets on and off. When I put my hand in it, it felt warm, not "neutral". My wife, who voluntereed as an unbiased subject (she didn't know what the temperature was), also reported that the water was warm.
I can't forgive Verne for not having the common sense to test his idea before putting it in his book. Maybe he didn't have a thermometer.
1. Some editions, for example the French text at Gutenberg, give this name as Cyrus Smith.
2. The French text gives the temperature as “quatre-vingt-quinze degrés Fahrenheit (35 degrés centigrades au-dessus de zéro)”.
3. It should, of course, be “It’s”. This simple error is present both in my paperback copy and in the Gutenberg e-text.