an instrument for measuring the Density of the Air. 87 



The following table contains the last series of observations 

 which I have made previous to the instrument being finished, 

 with some additions. The thermometer inclosed in the 

 lanthorn was a good one, but not the one intended to be used, 

 which was not ready ; it was read with a reading microscope, 

 as was also the aerometric balance. The instrument was 

 placed in an upper room facing the west, in which no fire was 

 lighted. The barometer was a good aneroid one, with an 

 attached thermometer. I have not thought it necessary to 

 give the readings of these, but only the height of the baro- 

 metric column reduced to a temperature of 32°. 



The column of the densities is calculated from the formula 



with unity as the density for 32° temperature and 30 inches 



barometric pressure, taking a= — - for comparison with the 



aerometric balance. Although the changes take the same di- 

 rection, that is, greater buoyanc} 7 is indicated when there was 

 greater calculated density, yet the changes are not propor- 

 tional; and on comparing different parts of the series, we see 

 remarkable differences. Observations of the wet bulb ther- 

 mometer might have enabled us to explain some of these, by 

 the hygrometric changes going on; but if any remained, they 

 would indicate changes in the composition of the air of the 

 room. 



When Messrs. Watkins and Hill have got the construction 

 of the aerometric balance perfected, I should feel great interest 

 in the results obtained in different parts of the country, but 

 must leave the observations to others with better opportunities 

 and more time, and rest satisfied with having completed the 

 less important task of designing and superintending the con- 

 structions of the means of observation. 



The observations of the table have a point of interest in 

 having been taken through one period of storm, and another 

 of dark but high fog. 



