VOL. LXVII.J PHILOSOPHICAL TltANSACTIONS. 237 



exceedingly well with each other for the mean heat of the air, when the baro- 

 meter will come most frequently into use. 



The experiments in the 4th class are all that now remain tt) be mentioned. 

 The bare inspection of them show how greatly superior the elastic force of moist 

 is to that of dry air. It is true indeed that 2 kinds of irregularities present them- 

 selves among the results: 1st, with regard to the total expansion for 212°: and 

 2dly, as to the greatest exertion of the elastic force, which sometimes seems to 

 have taken place before the air has acquired the heat of boiling water. The 1 st 

 is easily accounted for: it must have arisen from different proportions of moisture 

 being admitted into the same quantity of air, which there was no possibility of 

 ascertaining, the bulbs and their apertures being of very different dimensions. 

 With regard to the 2d irregularity, it may have proceeded from error of obser- 

 vation, it being difficult to determine the accurate temperature near boiling; 

 especially when any part of the air rose above the top of the vessel, which was 

 sometimes the case, notwithstanding its extraordinary height. Be that as it may, 

 a very uniform increasing progression is perceived to take place, from the zero of 

 Fahrenheit, as far as 152° or 172°; and even to the boiling point, in those 

 esteemed the best experiments. By adhering to the mean result it will appear 

 that air, however moist, having that moisture condensed or separated from it by 

 cold, its expansion differs not sensibly from that of dry air. Thus the rate for 

 32° below freezing 2.22799, is nearly the same as in dry air; but no sooner does 

 the moisture begin to dissolve and mix with the air, by the addition of 20" of 

 heat, than the difference is perceptible: for instead of 2.46675, the rate for 20° 

 above 32° in dry air, we have 2.588 for that which is moist. In the next step of 

 20°, the rate for dry air is 2.3809; whereas that for moist is 2.97- In this 

 manner the progression goes on continually increasing, so as to give 7-86854 for 

 the mean rate on each degree of the 212°, which is near 3-i- times the expansion 

 of dry air. And lastly, the rate for the 20° between I92" and 212°, is 2-1- the 

 mean rate, and about 9 times that which corresponds to the zero of the scale: 

 but if the comparison be drawn from the mean of the 5th, 6th, and @th experi- 

 ments, as being probably nearest the truth, the total expansion of moist, will be 

 more than 4 times that of dry air; and the rate for the temperature at boiling, 

 will be nearly 1 5 times that which corresponds to the zero of Fahrenheit. 



Sect. 3. An Account of the Barometrical Observations made in Britain, — I 

 am now to give an account of the principal barometrical observations that have 

 been made in Britain, on heights determined geometrically with great care. 

 These heights are classed in the following list in 6 sets, according to the districts 

 of the country where they are situated, and nearly in the order of time in which 

 the observations were made. 



