102 ‘ REPORT—1843. 
Description of a Process for preventing the deleterious effects of Dry 
Grinding. By J. P. GRoLyer. 

Mr. H. Henessy read a paper on a very simple apparatus for the purpose of deter- 
mining the distance of objects. A moveable base line carries on it a small arc, by 
which the approximate determination of distances of 500 feet is easily effected. 
Omitted in the Report for 1842. 
On the Buoyant Floatwater. By Capt. A. W. Suricu. Read at Man- 
chester in 1842. 
Captain Sleigh stated the basis on which the buoyant sea-barrier which he has de- 
vised depends for its successful operation, is the fact that the agitation and drift of the 
sea is entirely superficial, and that the reaction, usually denominated the ground 
swell, caused by a current passing over an uneven bottom, is so extremely limited 
in its uppermost effect, that it cannot be detected when a hand lead is lowered within 
three feet of an irregular ground over which a tide passes rapidly (say five knots 
an hour). 

On the Barometric Compensation of the Pendulum. By Dr. Roxinson. © 
(Section A.)* 
At the Manchester Meeting of the Association Professor Bessel made a communi- 
cation on the improvement of the astronomical clock, which, with other valuable mat- 
ter, contained a proposal to compensate for the changes of rate produced by the vary- 
ing density of the atmosphere. ‘This appears in the Report of the Sectional Proceed- 
ings, and also at much greater length in No. 465 of the ‘ Astronomische Nachrichten.’ 
At the time Professor Stevelly remarked, that I had not merely proposed but applied 
this compensation twelve years ago +; and I should not have reverted to it, but that I 
think my method possesses certain advantages over that proposed by the illustrious 
astronomer of Kiénigsberg, which entitle it to the preference in practice. It was long 
believed to be demonstrated that the rate of a pendulum was influenced by the air’s 
density only as far as it lessened the are of vibration and diminished its gravity by 
buoyancy. The researches of Kater on the length of the second pendulum are all 
vitiated by this mistake, which was discovered by Bessel during a similar investigation, 
in which he found, by using balls of different specific gravities, that the received 
buoyancy correction is too small. As early as 1825, and without any knowledge of 
what Bessel was doing, I had ascertained the same fact by comparing the rates of my 
transit clock with the barometric indications; and Colonel Sabine gave the final proof 
of it by swinging the pendulum in a vacuum apparatus in the year 1829, The amount 
of it is far from inconsiderable; even with the mercurial pendulum of my transit 
clock, which weighs 21 pounds, and presents a very small surface, it is 0°36 for an 
inch change of the barometer. Now the remedy for this is obvious. If we attach a 
barometer to the pendulum, its fall transfers a cylinder of mercury from a point near 
the axis of motion to a greater distance from it; the time of vibration may thus be 
made to increase by the same amount that it decreases in consequence of the dimi- 
nished density of the air. In the expression of this increase, there are two disposable 
constants, the diameter of the tube and the distance of an extremity of the barometric 
column from the suspension of the pendulum. Two conditions may therefore be ful- 
filled. Bessel assumes as one, that the lower surface of the mercury shall be at the 
centre of oscillation ; an arrangement which does not seem to possess any peculiar ad- 
vantage, as he makes the compensation by giving an appropriate diameter to the tube. 
This however does not admit of adjustment, while the method adopted by me of shift- 
ing the barometric column makes it perfectly easy, and permits me to apply the other 
constant to correct the variation of arc produced by a change of resistance. It is ob- 
* This paper as given on p.17 is incomplete. 
+ Astronomical Memoirs, vol. v. Dependence of Clock’s rate on Barometer, 

