330 On the common Barometer. 



there is no aperture delineated, and it might be conceived 

 hermetically sealed. Mr. Leslie has introduced a cork, 

 and his figure certainly differs from those in the two edi- 

 tions of Van Helmont which are found in the library of 

 the College of Edinburgh, and which are perfectly alike. 

 Fig. 4. is copied from the edition of 1652. 



Mr. Leslie has not noticed a sentence immediately fol- 

 lowing that to which he has referred, in which the prin- 

 ciple of the diflerential thermometer is described. I shall 

 supply his deficiency : " Applicatur vasi A aliquis calor, ex- 

 cedens temperietn arabientis : tunc enim aer inckisus, se 

 dilatabit, secundum plus ant minus caloris, ac juxta, 

 alque excedit veram aeris temperiem clausl in vase D, con- 

 tra quern propellens aquam BC destruet cequilibrium per 

 iiimiam actionem. Ita quod in D erit comprcssus et con- 

 densatus aer, per restrictionem : ut cedat dilatationi factae 

 in A." 



It appears from these facts that Sir H. Davy is fully 

 justified in saying that a curious instrument like the dif- 

 ferential thermometer is described by Van Helmont, 

 *' which appears to have been the first in which the expan- 

 sive power of heated air was exhibited by its action upon 

 cold air." 



I shall not pretend to determine whether Mr. Leslie has 

 rot read this last passage, or whether he has read it and 

 has not thought proper to quote it: nor shall I offer any 

 observations on insinuations so unwarrantably and unjustly 

 brought forward by the Frotissor : as the ground for them 

 has been removed, they will tall of course. 

 I am, sir, yours, &c. 



JEdinburgii, Oct. 20, 1312. A. B. 



LVIL Tabular Corrections for the Rise and Fall of the 

 Mercury in the common Barometer. By E. Walkek, 

 Esq. 



To Mr. Tilloch. 



Sir, INo meteorological instrument is in higher estima- 

 tion than the Barometer. But barometers of the common 

 construction, with an upright tube and open cistern, are 

 not sufficiently correct for investigating the laws of meteo- 

 rology with the greatest precision, in consequence of the 

 quicksilver's rising or falling in the cistern with every va- 

 riation in the pressure of the atmosphere. In the best 

 barometers, however, this iniperi'eclion is corrected by 



soma 



