218 On the Horary Oscillations of the Barometer at Rome. 



if just sufficient to freeze the water under the njost favourable 

 circumstances, the contents of a vessel not fully exposed to 

 the influence of the sky may remain fluid throughout. I have 

 never found it practicable to make ice (operating, however, on 

 a small scale) when the temperature exceeded 4P on the level 

 of the pits ; but on such occasions the temperature is much 

 higher at some distance from the ground, and I have found a 

 series of bottles filled with water, suspended from a mast of 

 about seventy feet high, to show an increase of about 1^ for every 

 ten feet in height. Sir H. Davy is therefore right in saying that 

 ice may be made when the thermometer is above 50°, if he 

 allude to the upper regions of the air, or hills of moderate 

 height ; but, as I have already said, it cannot, according to my 

 experience, be made when the thermometer, suspended at the 

 distance of three feet from the ground, on the level plain, 

 stands at above 41°. I wrote a letter on this subject to Mr 

 Colebrooke some years ago, of which, I believe, I can still send 

 you a copy. My experiments extend to the height of 3400 

 feet, at which elevation, on a detached mountain, the tempera- 

 ture of the air at sunriseas several degrees higher than it is in 

 the plain of Bengal. I should like much to hear of this experi- 

 ment being repeated, or of its having been already tried in 

 some of the colder countries, where I suspect the same result 

 will be obtained in the winter season with a clear sky, and at 

 day-break. 



NuNGKLow, 1 0th November. 



Art. V. — Memoir on the Horary Oscillations of the Baro^ 

 meter at Rome. Communicated by the Author. (Continued 

 from p. 129.) 



The observations being reduced into the above table, it re- 

 mained to be considered how the actual results would be most 

 simply and accurately elicited. Had the observations been 

 complete for every hour of each day the means for each hour 

 in inches would have given the desired products ; but as we 

 find them unequally scattered through different days, they are 

 so strongly affected by what the positive height of the mercury 



