071 the Horary Oscillations of the Barometer at Rome. 1 15 



remarks on the construction of the instrument, and the pre- 

 cautions taken in the use of it. 



The barometer was made by myself, and was on Sir Henry 

 Engelfield's construction. I here give the note of its dimen- 

 sions. 



Inch. 

 Tube, internal diameter, - - 0.150 



external diameter, - - 0. 22 



Cistern, area for rise of mercury, - 9216 — 484 = 8732 

 But, internal area of tube, - 225 



Quotient or correction of cistern level, qqIsI 



The correction of level then being ^^ittj I have always in 

 practice considered it ^^o* The tube was of Roman manu- 

 facture, much thinner than the English ones, and I fear not 

 so perfectly equal. It was boiled over a spirit-lamp with the 

 utmost care, and ever after exhibited the most brilliant flashes 

 of electrical light in vacuo. The cistern was close and of 

 hard wood, but with a piece of cork inserted in the bottom to 

 admit the air ; and although, after being much carried about, 

 for a time it did not perform well, yet hung as it was constants 

 ly during these experiments in the sam^ part of the same 

 room, I have no reason to believe that it was the least slug- 

 gish in its variations. Attached to the case was a small ther- 

 mometer graduated by myself with the Fahrenheit and Centi- 

 grade scales ; the former has always been used. I likewise gra- 

 duated the inches of mercury on a slip of brass fixed to the case, 

 and a vernier, now on the tube, which showed to jj^ of an 

 inch. Though I cannot pretend that my graduation was all 

 to that degree of exactness, yet it certainly read off much 



raised to that standard of exactness of which there can be no doubt that it 

 is susceptible, but from which it is to be lamented it is at present so for 

 removed." In another place, " Observers will render a much greater 

 service to science by devoting less of their time to the inspection of their 

 instruments, and more to applying the proper corrections." Should that 

 intelligent philosopher ever read these pages, I think I can assure him that 

 few observations have ever received more attention than mine with regard 

 to the boiling of the mercury in the tube, the accuracy of the observations, 

 and the precision of the corrections. 



