110 Proceedings of the British Association. 



Sir D. Brewster then gave a provisional report on the hourly 

 Meteorological Observations carried on at Inverness, at the expense of 

 the British Association, by Mr. Thomas Mackenzie. It appeared from 

 this report that the hours of mean temperature for the whole year 

 were 8h. 30m. and 7h. 35m. the interval between which is llh. 15m. 

 which is called the critical period, and which is a constant quantity. 

 This value of the critical interval at Leith was found to be 1 lh. 15m, 

 which was the average of some years' observations ; and it is inter- 

 esting to observe, that the very same amount of the critical interval 

 has been found at Inverness, in a much more northern latitude. 



The President remarked, that Meteorological observations requir- 

 ed the atmosphere to be in a very peculiar state, in order to insure 

 accurate definition. He believed it was Struve who first remarked, 

 that whenever the temperature of the night sunk much below the 

 mean of the preceding 24 hours, no accurate definition of objects was 

 to be expected. Dr. Robinson said, the hygrometrical state of the 

 air was of much consequence for astronomical observations ; he found 

 that it required to be very near the point of saturation, as a difference 

 between the wet and dry bulb thermometers of more than a degree 

 or two precluded all accurate definition, the brighter stars having 

 then a tendency to throw out scintillating lines ; and it was only in 

 the moist state of the air that they appeared distinct in themselves, 

 although surrounded by faint coloured rays. Rev. Dr. Scoresby 

 remarked how one branch of physical research bore upon other 

 branches. He believed that the hour at which Lord Rosse found 

 that he could best test the accuracy of figure given to his splendid 

 reflectors was that which Sir David Brewster had ascertained to be 

 the time of mean temperature for the 24 hours. Lord Rosse said, 

 that the test being the formation of a distinct image of a watch-dial 

 placed at a considerable elevation, say 100 feet, above the tower, it 

 was necessary, in order that there should be no tremor of the air, 

 that the temperature within and outside the tower should be as nearly 

 equal as possible, and that this was pretty much the time of the mean 

 temperature of the day — about 20 minutes past nine in the morning. 



' On the Analogy of the Existencies or Forces, Light, Heat, Voltaic 

 and ordinary Electricities,' by John Goodman. The existencies, 

 light and caloric, having by the labours of M. Melloni, Dr. Forbes, 



