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.
