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XLVt. On the New Mountain Barometer. By Sir Henry 

 C. Englefield, Bart. F.R.S. and F.S.A. 



To Mn Tilloch. 



Sir, JL he experience of three years having ascertained the 

 convenience and utility of the mountain barometers, made 

 on the principles of which a description, drawn up by me, 

 was inserted in your Journal, (vol. xxx. p. 46,) I am induced 

 to address you again on the subject ; both to inform the public 

 ~f some improvements made in their construction since my 

 former letter, and to propose some mode of collecting, for 

 general^enefit, the observations made by individuals. 



The improvement in the construction is principally hi 

 the cistern. It had been found that when exposed to great 

 motion in an unfavourable position, which in long journeys 

 is not easily avoided, the agitation of the mercury had 

 several times cracked the tube towards the top, in a fissure 

 scarcely perceptible to the eye, yet sufficient to Jet in slowly 

 a small portion of air. To remedy this inconvenience, the 

 cistern has now a bottom of leather on which a screw presses 

 in the usual mode, so as to force the mercury nearly to 

 the top of the tube when packed for carriage. This screw 

 is to be unscrewed as far as it can, when the barometer is 

 prepared for use; and the leather bag is so adjusted, that 

 there is no reason to fear that the capacity of the cistern 

 thus unscrewed for use, will ever be sensibly different from 

 itself at different times. It may be just mentioned, that 

 when the barometer is carried by a careful person, it is by 

 no means necessary to screw up the bag between every 

 station; as, when unscrewed, the instrument is in precisely 

 the same state that it always was, in those of the first con- 

 struction*. 



Mr. Jones, at the desire of several gentlemen, has en- 

 deavoured to add a gauge point and adjustment to keep the 

 mercury in the cistern ever to the same height, as -in other 

 mountain barometers, but such addition has been found in 

 practice productive of more inc onveniencc than advantage. 

 He now, therefore, measures the content of every tube se- 

 parately, and engraves on the mounting the correction to 

 be made to the results, as stated in the former paper; and 

 by this method it is presumed that all errors from the 

 want of a gauge point must be prevented. Mr. Jones has 



* The screw which frees the cistern for use, is protected by an outer cap 

 from being spoiled by idle curiosity, or irjurcd by a blow, which often 

 happened to those barometers where this screw was unprotected. 



Vol. 36. No. 150. Oct. 1810. Q now 



