292 On the construction of the Barometer, ^c. 



Art. XI. — On the construction of the Barometer, and other Phi- 

 losophical Instruments in this country ; by James Green, Philo- 

 sophical Instrument Maker in Baltimore. 



TO PROFESSOR SILLIMAN. 



Dear Sir — Early after the publication of No. 1. Vol. xxvii of 

 your very valuable Journal, my attention was directed to an article 

 on the construction of the Barometer; and although interested in all 

 improvements of the means of making philosophical observations,! felt 

 particuarly so in the present instance, from having received an order 

 on account of the public service to construct a Barometer, that would 

 furnish accurate indications of atmospheric pressure on any plan 

 which my experience in this branch of manufacture might approve. 



In perusing the article I was particularly attracted by the coinci- 

 dence which existed in the construction of some parts of Mr. Du- 

 rant's instrument and a modification of Gay Lussac's Barometer by 

 M. Bunten, a favorable report of which was read before the French 

 Institute, by Messrs. Savart and Arago, and which subsequently receiv- 

 ed a favorable notice from Baron Humboldt, who after having used the 

 instrument during a travel through Germany of several months, tes- 

 tified to its efficiency in preventing the admission of air into the mer- 

 curial column. The Globe Portable Barometer of Mr. Durant 

 seems to present all the advantages derivable from that arrangement 

 with some additional securities. However, I must freely confess 

 that the gratification which a Philosophical Instrument Maker might 

 be presumed to derive from such a circumstance was in some meas- 

 ure diminished by discovering the writer to be under the impression 

 that there existed an indifference to all efforts at the perfection of so 

 important an instrument as the Barometer, on the part of the manu- 

 facturer, who I hope may be often found both " able and willing " to 

 resort to all the precautionary means necessary to insure as perfect 

 an instrument as practicable, when such an instrument is wanted. — • 

 In so far as the accuracy of the instrument may depend upon the 

 boiling of the mercury in his tube, it is a very common thing with 

 him to take the " patient care " required in this operation. Indeed, 

 it has been my uniform practice to observe this precaution in all in- 

 stances, where such accuracy was desirable, and also to use mercury 

 that had been redistilled. 



I have been induced to state these facts, because from the tone of 

 the article alluded to, an individual who might desire a perfect instru- 



