of the perpendicular, or nearly two degrees. With a mercurial pendulum the arc of semivibration must be 1° 31′ 44′′, or about one degree and an half.

And it is an extremely curious circumstance, that without any reference to the attainment of this balance between opposite disturbing causes, our best clocks should have been fortuitously made to vibrate very nearly in the arc, which reduces them to an equality.

Friction is supposed to be so very small in amount, and moreover to possess such very trifling influence on these minute changes as not to require attention.

All thermometrical changes are corrected by the ordinary compensation. Nor is it of any practical importance to estimate h, (the height of the barometer from whence the variation i. is to commence,) at any other standard than 30 inches, unless, indeed, the barometer should be placed in some very elevated situation.-When other tables should be constructed from the new values of t and s.

The following table, reprinted from the former communication, gives the effect of buoyancy on a brass pendulum. These numbers multiplied by six-tenths, (.6) become adapted to a mercurial pendulum; and the same numbers multiplied by the following factors, with their signs changed, will give the effect arising from variations of the circular excess; for degrees, and quarters of degrees of semivibration.

A TABLE, reprinted from the XVth vol. for correcting the time, as shewn by a clock, having a brass weight, or ball to its pendulum, for the variation of one inch in the height of the barometer. ARGUMENTS. The time elapsed since the last observation of the barometer.

±
2

And the present observed height - 30 inches the variation between the observations

Additive, if the sum is Plus-Subtractive, if it is Minus.

5

6

7

8

10 10 10 10 to to to 10 10 8

888

1228

01

01

01

01 01 01

01 01 01 01

02

02 02

02 03

02 02 02

03

03

03

02 03 03 03 04

04

04 05 05

24 0204 06 08 10 12 14 16 18 20

In the case of a mercurial pendulum, these quantities must be reduced to three-fifths (.6) of their magnitudes in the table.

ART. XII. Correction of an Error in the "Meteorological Essays." By J. F. Daniell, F.R.S.

I AM indebted to an anonymous critic in the Dublin Philosophical Journal, no less for pointing out a mistake into which I have carelessly fallen in my Meteorological Essays, than for the flattering terms in which he has spoken of the work in general. The mistake occurs at p. 178, & seq. in the table for finding the specific gravities of various mixtures of air and aqueous vapour. The table consists of five columns ;-the first contains the degrees of temperature from 0° to 90°; the second the alterations of volume due to each degree of heat; the third the increments of volume from the addition of vapour of any given temperature; the fourth the increase of density from the weight of the same vapour; and the fifth the correct specific gravity of saturated air. In calculating the last I have deducted or added the increase of volume from unity, instead of calculating the specific gravity in inverse proportion to such alteration. The mistake is so very obvious, that I could not have extended the table much further without having been aware of its commission: but within its actual limits it is of much less amount than might have been at first supposed. The facility with which errors of this description slip from the pen cannot be better illustrated than by an oversight of an analogous nature of my Reviewer himself, who observes, upon the formula for calculating the expansion of gas from moisture, "If the volume of the dry gas 1, and the height of the barometer and the elasticity of steam be represented respectively by p and ƒ, when the gas is saturated, its volume will have become = Now if this were the case, when the

Р

p-f

height of the barometer is 30, and the elasticity of the vapour is

30 also, the volume becomes

30 30-30

which is clearly impos

sible; the formula upon which my calculation is founded is

p+£; which gives, upon the same supposition, 30+30

the volume.

or double

30

The following corrected table should be substituted for the one already alluded to.

-.00624

+.00647

+.00411

1.0041

45 +.02708

+.01134

+.00697

.9699