390 NOTES OF A NATURALIST. 



mann * has proposed a formula for the reduction of barometric 

 observations which implicitly assumes that the rate of decrement 

 of temperature in ascending mountains is uniform, inasmuch 

 as he takes the mean of the temperatures observed at the 

 higher and lower stations as the value of the mean tempera- 

 ture of the column of air between the two stations. It would 

 appear that his adoption of the hypothesis of an uniform rate of 

 decrease is merely due to the apparent impossibility of discover- 

 ing a more satisfactory^ hypothesis. Following on a line of 

 inquiry first suggested by the late M. Plantamour and M. 

 Charles Martins, Dr. Riihlmann has analyzed a series of two- 

 hourly observations of temperature made during six years at 

 the hospice of the Great St. Bernard and at the Geneva 

 Observatory. Treating the mean temperature of the column 

 of air between the levels of those places as the unknown 

 quantity, and neglecting, as unimportant, the corrections for the 

 tension of aqueous vapour and for gravity, he has deduced the 

 " true temperature," as he styles it, of the intermediate column 

 from the equation of condition between the pressures, the 

 heights, and the temperatures of the two stations, for the average 

 of the two-hourly periods of observation for each month. He 

 has shown that, while on the average of the entire year the 

 mean "true temperature" of the intermediate column of air 

 agrees pretty well with the mean of the yearly observations at 

 the two extreme stations, the means for the separate hours and 

 those for the separate months usually differ widely from the so- 

 called "true temperatures" for the corresponding periods. 



From this investigation Dr. Riihlmann has shown that during 

 the warm hours of the day, and the summer months, the "true 

 mean temperature" is lower than the mean of the observed 

 temperatures at the two extreme stations, while at night, and 

 during winter, it exceeds that mean to a rather greater extent. 

 It may be objected that the cause of the apparent discrepancy 

 lies in the fact that, in thermometric observations, we obtain, 

 not the true temperature of the surrounding air, but that of the 

 thermometer, and that, however carefully screened, the thermo- 

 meter cannot be completely freed from the effects of radiation 



* See " Die Barometrischen Hohenmessungen und ihre Bedeutung 

 fiir die Physik der Atmosphare," Leipzig, 1870, by R. Ruhlmann, 



