On the Barometer, 467 



do not measure the air until after we have determined with 

 the usual precautions the temperature and pressure to 

 which it is exposed. 



We must only introduce in this manner into the mano- 

 meter, a liquid, which most commonly does not disturb the 

 results, and the influence, of which we can always ascertain : 

 if we were afraid, however, that it would interrupt the ex- 

 periment, we might receive it into a vessel disposed for this 

 purpose in the inside of the manometer. 



Fig. 3 shows the various pieces just described, ready to 

 be adjusted : fig. 5 is a section of these same pieces all 

 adjusted. 



We ought to take care, in the construction of this appa- 

 ratus, to give the hole of the key of the stop-cock a dia- 

 meter sufficiently large to admit of the easy no win j of the 

 waAer of the tube, and it ought not to be less than twelve 

 millimetres. In order that the air contained in this hole 

 may be in-thesame circumstances with that which occupies 

 the whole capacity of the manometer, we leave the stop- 

 cock open during the experiments, as seen in fig. ,1 and 2 ; 

 we intercept the communication with the external air by 

 means of a copper stopper O (fig. 1 and 4) which has the 

 same screw with the mounting of the divided tube, and 

 which is also furnished with a run of leather. In order to 

 close it properly, it has at its surface a square cavity which 

 is seen at p, into which we insert the stalk r of the same 

 form which is at the extremity of the handle of the key T. 

 We then only close the stop-cock at the moment when we 

 wish to extract the air from the manometer. 



LXXXU. On the Barometer. By Richard Walker, Esq. 



To Mr. Tdloch. 



Sir, I^onsidering that I may not have been sufficiently 

 explicit in my last paper, respecting the effects of the 

 difference of temperature on the weather, 1 have been in- 

 duced 'to offer the following remarks on that part of the 

 subject. 



A warm temperature of the air, at any degree of density 

 of the atmosphere, will retain a greater portion of water in 

 a state of chemical combination, than a cold temperature of 

 the air at a similar degree of density of the atmosphere. 

 Hence we may account for the almost constant dry state 

 »f the lower stratum of the atmosphere during the sum- 



2 G 2 MEK 



