Thermometers have hitherto been defective for meteorological purposes, in so far as they only point out the degree of heat at the moment of inspecting them, but do not show what the difference of temperature has been, from the time of one observation to that of another: Nor has any instrument been yet constructed, so far as I have been able to learn, which will record the intermediate degrees of heat.

The ingenious Robert Hook, in the end of the last century, mentions his intention of making a thermometer for the above purpose ; but it does not appear that it was ever executed: Neither does he explain how it was to have been done.