133 



schel, accompany this paper : one, a sketch of his telescope at 

 Slough, fixed from its image in a lens ; and the rest copies of engra- 

 vings and drawings, some reverse, or first transfers ; and others se- 

 cond transfers or re-reversed pictures. 



March 21, 1S39. 



The MARQUIS of NORTHAMPTON, President, in the Chair. 



Thomas William Fletcher, Esq., and the Rev. Thomas Gaskin, 

 were balloted for, and duly elected into the Society. 



The following papers were read : — 



I. "Description of a Compensating Barometer, adapted to Me- 

 teorological purposes, and requiring no corrections either for Zero, 

 or for Temperature." By Samuel B. Howlett, Esq., Chief Military 

 Draftsman, Ordnance. Communicated by Sir John F. W. Herschel, 

 Bart., K.H., V.P.R.S., &c. 



In the instrument here described, there is provided, in addition to 

 the ordinary barometric tube (inverted, in the usual way, in a cistern 

 of mercury,) a second tube of the same dimensions, placed by the 

 side of the former, and likewise filled with mercury, but only to the 

 height of twent}^- eight inches above the level of the mercury of the 

 cistern. This tube is closed at its lower end, and fixed to a float 

 supported by the mercury in the cistern : and it bears, at its upper 

 end, an ivory scale, three inches in length. The elevation of the 

 mercury in the barometric tube is estimated by the difference be- 

 tween its level and that of the mercury in the closed tube ; and is 

 measured on the ivory scale by the aid of a horizontal index, em- 

 bracing both the tubes, and sliding vertically along them. As the 

 float which bears the closed tube, to which the scale is attached, 

 rests freely on the mercury in the cistern, and consequently always 

 adjusts itself to the level of that fluid, no correction for the zero 

 point is needed ; and as every change of temperature must similarly 

 affect the columns of mercury in both the tubes, after the scale has 

 been adjusted so as to read correctly at any given temperature, such 

 as 32°, which may be effected by comparison with a standard baro- 

 meter, every other reading will correspond to the same temperature, 

 and will require no correction. The author considers the error 

 arising from the difference of expansion corresponding to the dif- 

 ferent lengths of the two columns of mercur\% and which will rarely 

 amount to one four-hundredth of an inch, as too small to desen^e 

 attention in practice, being, in fact, far within the Hmits of error in 

 ordinary obser^-ations. 



Subjoined to the above paper is a letter from the author to Sir 

 John Herschel, containing a statement of comparative observ'ations 

 made with a mountain barometer, and with the compensation baro- 

 meter, from which it appears that the use of the latter is attended 

 with the saving of a great quantity of troublesome calculation. The 

 comparative observ^ations are given in a table, exhibiting a range of 

 differences from -^'012 to —'016 of an inch. 



