164 Prof. Potter on the Relations of the Elastic Force, 



atmosphere at the further end, around the stem of the thermo- 

 meter ; then if the stopcock were opened for a short space of 

 time, say not exceeding one second, the air in the chamber being 

 replaced many times in this interval, then by watching the mer- 

 cury of the thermometer with a compound microscope it would 

 be easily seen whether it rose or fell, which was all I proposed 

 to learn from it ; for I intended to bring the chamber and its 

 contained thermometer to a temperature, before putting it in its 

 place at each trial, which I wished to compare with that of the 

 issuing jet ; and by successive trials I expected to find the tem- 

 perature at which the jet produced no effect on the thermometer, 

 and then its temperature would be determined. 



In this mode of use, as a thermoscope only, the common mer- 

 curial thermometer is very sensitive, as the following shows. 

 When the difference of temperature of the jet and thermometer 

 exposed to it was several degrees, the first impression of the jet 

 appeared the contrary of what took place the instant afterwards 

 when the mercury became heated or cooled. The first impres- 

 sion was evidently due to the heating or cooling of the very thin 

 glass of the bulb, which caused an indication the opposite of 

 what occurred when the contained mercury became affected with 

 the change of temperature. In most, though not all, of my 

 observations, I noted the first as well as the second impression 

 of the jet, and also the tendency of the thermometer to rise or 

 fall after the jet had ceased, together with its slow continuous 

 rise to the temperature of the siirrounding objects afterwards. 



The early trials I made this spring convinced me that I should 

 not be required to go to much expense for freezing mixtures, to 

 reduce the exterior thermometer to the temperature of the issuing 

 jet of air ; but I found results so widely different from Poisson's 

 assumption, and so small in comparison with it, that I at first 

 doubted the adequacy of my method of experimenting for deter- 

 mining the temperature of the jet in given circumstances of 

 pressure and temperature within the receiver. From professional 

 duties having to discontinue the experiments, I had time to think 

 over the improvements required in order to determine such small 

 changes of temperature with an accuracy sufficient to determine 

 their law. 



On resuming the experiments this summer, I had a fine silk 

 thread stretched across the field of view of the microscope, which 

 was afterwards changed for fine cross micrometer lines at right 

 angles, and which gave great additional precision by bringing 

 the image of the extremity of the mercury in the stem to very 

 near apparent touching of the cross line, when the stopcock was 

 opened. In this way a change of T ^th of a degree Fahrenheit 

 was distinctly visible. When it was found that tenths of degrees 



