VOL. LXVII.] PHILOSOPHICAL TRANSACTIONS. 233 



of the whole. Thus, if the weight of the atmosphere should at any time be so 

 great as to sustain 31 inches of quicksilver, the equation for difference of tem- 

 perature will be just -jV part more than that for 30 inches; at 25 inches it will 

 be {.; at 20 inches -f-; at 15 inches 4; and at 10 inches only -i- of that deduced 

 from experiments; and so on for any other quantity. 



Sect. 2. Experiments on the Expansion of -Air in the Manometer. — The ther- 

 mometer used in these experiments was above 4 feet long. Its scale extended 

 from — 4° to + 224° of Fahrenheit, each degree being more than -J^ of an 

 inch: when the barometer stood at 30 inches, its boiling point was fixed in the 

 tin vessel formerly described. Mr. Ramsden's thermometers generally rise in the 

 same vessel 2\3\°; and the long thermometer, being placed in the vessel he 

 makes use of to fix his boiling points, rises only to 210°. 



The manometers were of various lengths, from 4 to upwards of 8 feet: they 

 consisted of straight tubes, whose bores were commonly from -^ to V- of an 

 inch in diameter. The capacity of the tube was carefully measured, by making 

 a column of quicksilver, about 3 or 4 inches in length, move along it from one 

 end to the other. These spaces were severally marked, with a fine edged file, 

 on the tubes; and transferred from them to long slips of paste-board, for the 

 subsequent construction of the scales respectively belonging to each. The bulb, 

 attached to one end of the manometer at the glass-house, was of the form of a 

 pear, whose point being occasionally opened, dry or moist air could be readily 

 admitted, and the bulb sealed again, without any sensible alteration in its capacity. 



The air was confined by means of a column of quicksilver, long or short, and 

 with the bulb downwards or upwards, according to the nature of the proposed 

 experiment. Here it must be observed that, from the adhesion of the quicksilver 

 to the tube, the instrument will not act truly, except it be in a vertical position; 

 and even then it is necessary to give it a small degree of motion, to bring the 

 quicksilver into its true place; where it will remain in equilibrio, between the 

 exterior pressure of the atmosphere on one side, and the interior elastic force of 

 the confined air on the other. All the experiments were made when the baro- 

 meter was at, or near, 30 inches. When the bulb was downwards, the height 

 of the barometer at the time of observation augmented, and when upwards, 

 diminished by the number of inches of quicksilver in the tube of the mano- 

 meter, expressed the density of the confined air. Pounded ice and water were 

 used to fix a freezing point on the tube; and by means of salt and ice, the air 

 was further condensed, generally 4, and sometimes 5 or 6 degrees below zero. 

 The thermometer and manometer were then placed in the tin vessel, among 

 water which was brought into violent ebullition ; where having remained a suffi- 

 cient time, and motion being given to the manometer, a boiling point was marked 

 on it. After this the fire was removed, and the gradual descents of the piece of 



VOL. XIV, H H 



