On Ascertaining Mean Temperature, 85 



Art. VIL — On the Construction of an Instrument for Ascer^ 

 taining the Mean Temperature of any Place. By Dn. E. 

 Davey. 



To ascertain tlie mean temperature of the year, and especially 

 of particular months of the year, in different regions of the 

 world, is an object of prime importance in Meteorology, The 

 mean temperature of particular days is also of interest, though 

 much less important. 



The mean temperature of the year may he pretty nearly 

 inferred from observations on the heat of the earth, at a 

 certain distance, say ten feet, from the surface, where it is 

 almost beyond the reach of influence from the seasons ; and 

 the resvilts thus obtained will probably be almost exact if 

 taken at opposite seasons of the year. By this method, how- 

 ever, we are informed rather of the average temperature of 

 a succession of years, than of any particular year, and we are 

 by no means enlightened as to the differences of the seasons. 



The mean temperature of particular days maybe, of course, 

 ascertained by hourly observations on the thermometer, and 

 taking their mean ; and the temperature of the month would 

 be calculated from the mean of the days. This method is, 

 however, obviously too troublesome to be carried into 

 practice. 



It was remarked by Humboldt, from observations made In 

 France, that the temperature at sun-down, is in general 

 pretty nearly the mean of the highest and lowest of the 

 twenty-four hours : the lowest being at sun-rise, and the 

 highest, about two hours after noon. He preferred, however, to 

 take the actual mean of the two extremes. These extreme 

 points may be very exactly ascertained by means of the instru- 

 ment well known as Six's day and night or register thermome- 

 ter, which leaves a mark of the highest and lowest points which 

 the thermometer has attained since the last previous observa- 

 tion. 



The description of my sensitive thermometer pendulum has 

 reference to the drawing. It consists of a glass tube, fixed 

 upon any suitable frame, A being the axis of oscillation. 

 The glass tube is bent in the form of a reversed syphon, of 

 which each limb, but necessarily the limb G E F, is upwards 

 of thirty-two inches in length. It contains three bulbs, of 

 which the bulb B and part of C D down to D with the 

 intervening tube contains air. — The remaining portion of the 

 bulb C D and part of the bulb E F and intervening tube, 



