432 Captain G. Underwood on his plan for a Barometer. [Aprii, 



To the Editor of the Madras Journal of Science and Literature. 

 Sir — With reference to a communication fivom me relative to an iron 



self-registering barometer, I beg to inform you that a scientific friend 

 of mine at Ootacamund, who has evinced much interest in this matter, 

 and who was good enough to compare and register the indications of a 

 Yery rough iron one in my possession with his standard instrument, 

 pointed out to me a few days ago in the second volume of the supple- 

 ment to the Encyclopedia Britannica, a drawing of an iron barometer 

 proposed by Blondeau. 



As that instrument was of the syphon kind and was intended to act 

 exactly like the one proposed by myself (the self-registering apparatus 

 excepted), I hasten to assure you that I was perfectly ignorant of Blon- 

 deau's recommendation, and had previously made every inquiry in my 

 power as to former suggestions on this head, and, from a correspondence 

 with J. Prinsep, Esq. the Editor of the Calcutta Journal of Science, I 

 was led to believe that iron had neither been tried or suggested, and 

 would most probably be found defective. It does not appear that the 

 iron barometer, alluded to in theEncyclopeedia, was ever tried,andas the 

 self-registering portion of my project has not to my knowledge been 

 pointed out by former philosophers, \ shall now state that the experi- 

 ments I have made riith the roughly formed instruments manufactured 

 in Coorg, give me the most sanguine hopes of perfect success. The 

 iron has been found to be air tight, but the following improvements to 

 the drawings before sent seem to me essential tp the perfection of the 

 instrument. a 



At a, a small hole must be made for filling the instrument which 

 must contain mercury sufficient to keep its surface when reversed at 

 the dotted line c, to guard against the possibility of air passing up the 

 long limb when set to work. At b, a mercurial tight stop cock should 

 be placed taking care to leave ample room for the expansion of the 

 mercury, which however even in this country need not be more than 

 3- 10th of an inch, since about 30° of heat will increase the length of 

 the mercurial column in this barometer only l-10th of an inch.* 



I remain, Sir, 



Camp Neelgherries, January 2, 1837. Yours obediently, 



G. Underwood, Capt. of Engineers, 



Mr. Hodgson, of Nipal, in a letter received while the last pages of 

 this Journal were passing through the press, requests us to alter the 

 generic name of the Perdix described at page 303 from Arbor ophila to 

 Dendrophila. This emendation, we are sorry to say, arrived too late 

 to be introduced into the body of the work. — Editor. 



* This letter was received when the last number of the Journal was in the press, 

 Captain Underwood's article having been struck off. It might have been introduced 

 in a Postscript to that number, but was overlooked, for which wc beg to apologize to 

 the Author,— Editor x 



