there could be no light abroad at that hour, it follows that none could enter from without; and that of conſequence, my windows being accidentally left open, inſtead of letting in the light, had only ſerved to let out the darkneſs: and he uſed many ingenious arguments to ſhew me how I might, by that means, have been deceived. I own that he puzzled me a little, but he did not ſatisfy me; and the ſubſequent obſervations I made, as above mentioned, confirmed me in my firſt opinion.
This event has given riſe, in my mind, to ſeveral ſerious and important reflections. I conſidered that, if I had not been awakened ſo early in the morning, I ſhould have ſlept fix hours longer by the light of the ſun, and in exchange have lived ſix hours the following night by candle-light; and the latter being a much more expenſive light than the former, my love of œconomy induced me to muſter up what little arithmetic I was maſter of, and to make ſome calculations, which I ſhall give you, after obſerving, that utility is, in my opinion, the teſt of value in matters of invention, and that a diſcovery which can be applied to no uſe, or is not good for ſomething, is good for nothing.
I took for the baſis of my calculation the ſuppoſition that there are 100,000 families in Paris, and that theſe families conſume in the night half a pound of bougies, or candles, per hour. I think this is a moderate allowance, taking one family with another; for though I believe ſome conſume leſs, I know that many conſume a great deal more. Then eſtimating ſeven hours per day, as the medium quantity between the time of the ſun's riſing and ours, he riſing during the ſix following months from ſix to eight hours before noon, and there being ſeven hours of courſe per
