Secondly, Rey was the first to make use of the expansion of a liquid in the construction of a thermometer. In a letter written to Father Mersenne, 1 January, 1632, he said: "I observe there are diverse kinds of thermoscopes and thermometers; what you tell me does not agree with mine, which is merely a small round flask having a very long slender neck. To make use of it, I put it in the sun, and sometimes in the hands of a fever patient, having filled it quite full of water except the neck; the heat expanding the water makes it ascend by a greater or less amount according to the great or little heat."
This evidently describes a water thermometer, or thermoscope, and so far embodied a new principle, yet it was still influenced by the pressure of the air. The instrument did not attract much attention.
Mersenne himself devised a modification of the air thermoscope intended to increase its delicacy, and described it in a work published at Paris in 1644, four years before his death (Cogitata physico-mathematica). The instrument consisted of a narrow tube having a large bulb at one end and a small one at the other, the latter being pierced with a minute hole;