Nephelescope

A nephelescope is a device invented by James Pollard Espy to measure the drop in temperature of a gas from a reduction in pressure; originally used to explore the formation of clouds.

Air is pumped into the vessel until a desired pressure is reached, the stopclock is then closed and the temperature allowed to equilibriate.

One advantage of using negative pressure was that a glass vessel could be used, which allowed the observation of condensation and droplets resulting from the drop in temperature.

The nephelescope enabled Epsy to predict the change in heat of air as water vapor became cloud.

In other words, latent heat released from the condensation of water mitigated some of the cooling from expansion of moist air.