ON THREE BALLOON ASCENTS IN 1865 AND 1866. 373 



in the different seasons of the year ; that at about snnset the temperature 

 varies but very little for a height of 2000 feet ; that at night with a clear sky, 

 from the only experiment made, the temperature increases with increase of 

 elevation ; that at night with a cloudy sky there was a small increase of tem- 

 peratm-e as the height increased ; that in the double ascent on May 29, the 

 one just before sunset and the other after, it would seem that after radiation 

 is set in the heat passes upwards till arrested, where the air is saturated with 

 vapour, when a heat greater by 5° was experienced after sunset than at the 

 same elevation before sunset. 



Two years ago, when I exhibited the mean results of the experiments then 

 discussed, I did so with much confidence, and thought all that then was needed 

 •was to verify the results exhibited. jS'ow, with increased knowledge, I speak 

 very differently, believing that many more experiments are necessary, and 

 that they should not be confined to this countiy. 



Certain it is, from the very remarkable results obtained from the night 

 ascents, which might, with sufficient number of observations, have important 

 bearing both on the theory of astronomic refraction and on the theory of heat, 

 that nocturnal observations deserve repetition and extension, 



§ 3. Desceiption op the Table of Observations. 



All the meteorological observations taken during the ascents are contained 

 in Table I. 



Column 1 contains the times at which the observations were made. Column 

 2 contains observations of the barometer corrected for temperature and index 

 error. Column 3 contains the height above the level of the sea, as deduced 

 from the barometric readings in column 2, by the formula of Bailey, checked 

 at intervals by that of Laplace, which is as follows : — 



Z=log(|)x60159(l+^i^)(l + 0-002837cos2L)(l+yg|§^^ 



where Z is the height required, and h, h', t and t' the height of the barometer, 

 corrected for temperature, and the temperature of the air at the lower and 

 upper stations respectively, L the latitude. The temperature of the air for the 

 position of the balloon has been derived from the readings in column 4. 

 Columns 5 to 7 contain the observations with wet-bulb thermometer, its 

 depression beiow the readings of the dry-bulb, and the deduced dew-point. 



The Astronomer Royal had observations made every ten minutes at the 

 Eoyal Observatory, Green-ndch, on the days of ascent, by Mr. Nash of the 

 Magnctical and Meteorological Department. 



The height of Greenwich barometer cistern above the mean sea-level is 159 

 feet. 



