CLIMATES OF GREAT MALVERN AND LONDON. 217 



is necessarry to premise the conditions of the two instruments, 

 and the circumstances of the localities in which they are placed. 



In the Standard Barometer of the Royal Society the tube is 33|- 

 inches long ; its exterior diameter 0.86 inch, and the diameter of 

 the bore 0.530 inch. The cistern is turned in well-seasoned 

 mahogany, and there is a small cavity in its bottom to receive the 

 end of the tube, which rests upon it ; a groove communicates with 

 the cavity, to ensure the free passage of the mercury. Everything 

 has been studied in this instrument to render accuracy attainable 

 with as little trouble as possible to the observer. The diameter of 

 the tube renders the correction for capillary action almost unneces- 

 sary. The correction for the capacity of the cistern has been con- 

 trived to be J_ of the result above or below the neutral point 

 30.576. ^^^ 



The barometer at Great Malvern — on the accuracy of which the 

 following statements depend — is a very carefully constructed in- 

 strument. The tube is 34 inches long, its exterior diameter .5 

 inch, and the diameter of the bore .280 inch. The mercury has 

 been boiled throughout in the tube. The cistern is turned in 

 mahogany, and through the bottom the tube passes into a leathern 

 bag firmly fixed to its circumference -, a small hole forms a com- 

 munication between the cistern and the mercury in the bag : at 

 one part of the circumference of the cistern an ivory point is fixed, 

 and a screw acting upon the lower part of the bag will always keep 

 the surface of the mercury just touching this point : thus the 

 correction for the capacity of the cistern is rendered unnecessary. 

 The scale has been duly affixed after several accurate measure- 

 ments from the ivory point. 



In the following statement of barometrical heights, great pains 

 have been taken to apply every correction so that they are strictly 

 comparable one with another — in the Royal Society's barometer 

 the variable correction for capacity — and in the barometer at Mal- 

 vern, the constant one, .055, added for capillary action j the diameter 

 of the bore in the latter instrument being so much smaller than in 

 the former, renders this requisite. The whole of the heights 

 stated have been reduced to one temperature, 32° F.* 



* That the reader may be put in possession of the practical application of these 

 corrections we shall subjoin the detail of the process, taking the mean height of the 

 barometer for September 1834 in London and Malvern. 



London. Malvern. 



Mean Height. Mean Height. 



30.114 f Temperature 62.9— 29.441 ^ Temperature 59.7 

 — .005 > Correction for capacity obtained .055 >-Add correction for capillary 

 — — — \ tlms 30.576 neutral point \ depression 



30.109 30.114 29.496 



00462 ' ^ Correction for temperature 



—.083")^ *• f . . 29.425 True hei<^ht at 32° 



J> Correction for temperature ^yn -^ ajj ° * * ,• ^ 

 I ^ .570 ^ Add a constant correction for 



30.026 True height at 32° F. Vthe elevation of Malvern .570 



29.995 J and the results very nearly agree 



In order to make a comparison between two barometers absoluteltf accurate) it is 



