2. A Thermograph, for observing the temperature of the air.

3. A Hygrograph, for observing the temperature of evaporation. These instruments were furnished by Mr. Adie, of London, in 1861, to replace others of which the scales were such that the photographic traces inconveniently interfered with each other. (See Introduction to Meteorological Observations for 1861, page 6.)

The values of the divisions and of the zeros of the photographic scales for these thermometers were determined by an elaborate comparison of the results of the scale-readings with those of the dry and wet bulb thermometers, between temperatures ranging from 30° to 78° of the air-temperature. That the assumed values were accurate is shown by the following general comparison of the mean monthly results at 2h, 10h, and 22h with the mean readings of the dry and wet thermometers at those hours.

Comparisons of Thermograph and Hygrograph with Standard Dry and Wet Bulb Thermometers.

The Thermograph and Hygrograph are placed side by side on the same metal frame, and the case which holds the Camera and the whole apparatus is placed very near that containing the other thermometers, at a distance of about eight feet from the north wall of the west wing of the Observatory. For a detailed description of these instru

ments the volumes for 1854 and 1855 may be consulted. For these and all the other Photographical Instruments gas is employed.

4. An Anemograph, for registering continuously the velocity and direction of the Wind. A full description of this Instrument, accompanied by engravings, is given in vol. xvii (for 1856) of the Radcliffe Observations. It was originally mounted on a small building on the north side of the Observatory, (but at a considerable distance from it,) at a height of 22 feet from the ground, and there was placed side by side with it, for comparison, a small portable Anemometer on Dr. Robinson's principle, designed by Professor C. P. Smyth. Near the end of the year 1857 a small building was erected on the top of the tower, at an elevation of 110 feet above the ground, and, by placing there another of Professor Smyth's Instruments, and comparing its indications with those of the one below, Mr. Johnson found that, on the average, throughout the year 1858, the velocity at the top of the tower was greater than that at the lower station, in the proportion of 2:26 1. (Rad. Obs. vol. xviii. p. [xxiii.]) The Anemograph was

:

kept in its old position till August 9, 1858, when it was taken down, and, after some alterations had been made by Mr. Adie of the Strand, it was placed in its present position on the top of the tower.

For the verification of the ratio given above, by means of the observations which were made in 1858, and for the value of the scale of the large Anemograph, the Introduction to the Meteorological Observations for 1858 may be consulted.

A few accidents occurred, as usual, during the year to the Anemograph and the smaller Anemometer. On Jan. 11 one of the fans of the direction-vane caught against the horizontal wheel, and prevented its turning to the direction required by the change of wind. It was sent on Jan. 12 to be repaired, and was replaced, on Jan. 13. On Aug. 20 the small Smyth-Robinson Anemometer No. I was removed for repairs, and No. 2 was put in its place. On Dec. 18 one cup of the latter was broken off by the wind, and on Dec. 22. No. I was resubstituted.

The indications of the Smyth-Robinson Anemometers were read daily at 10h A. M. throughout the year.

There is also on the tower an Hyetograph, or Self-registering Pluviometer (described in vol. xvii. p. [xviii]); but its action was found to be imperfect, and no continuous records of it exist. In February and March attempts were, however, made to improve its action, and with tolerable success. Since that time it has been serviceable for records of the rain during great storms, which is its chief utility.

The last instrument to be mentioned is an Electrograph, mounted

by Mr. Adie, consisting chiefly of a long copper rod, which, up to 1861, terminated in a sharp spike pointing upwards above the tower. It gives its indications photographically by connexion with a straw Electrometer. As the indications of electricity by this instrument had been very unsatisfactory, the single spike was replaced by a bunch of copper points on July 10, 1861; and, since that time, the indications have been much more abundant. The rod was broken by the wind on Jan. 11, and replaced on Jan. 16. It was removed again for some alteration on Feb. 14, and replaced on Feb. 17.

Daily Results of Meteorological Observations.

The Tables under this title are intended to give as complete a summary of the mean daily elements of the weather as the size of the page will allow.

The indications of the Barometer and Thermometers, in the 2nd, 3rd, and 4th columns of the Table, are the means of the twelve daily readings of the Meteorographic Registers, that is, they represent the means of the two-hourly indications, from noon of one day to noon of the next. When the registers are imperfect, the means of the recorded readings are reduced to the mean for the day, by the application of the corrections for the horary changes which are given in vol. xviii, as deduced from three years' observations.

The highest and lowest temperatures in the shade, in the sun, and on grass, were observed with the Self-registering Instruments by Negretti and Zambra previously mentioned. The corrections due to the want of coincidence of their readings with the Standard have been applied to their readings wherever they are necessary.

The record of rain is that given by the Gauge on the ground, which is examined every day at 10h A. M.

The amounts of the horizontal motion of the wind are deduced from the observations made with the small Anemometer of Professor Smyth's construction on the top of the tower, on the assumption that the readings of the Instrument are to be divided by 56 to give the corresponding motion of the air in miles. (See Introduction to Meteorological Observations for 1858, page [7]). The directions of the wind are deduced from the twelve daily readings of the Anemograph. mean directions are deduced from the simple arithmetical means of the ordinates measured on the Photograph, and are afterwards converted into the corresponding points of the compass. At times when the Anemograph has failed, the estimation of direction is the mean of the three estimations made by the eye at 2b, 10h, and 22h.

The

The amount of cloud is estimated as usual from 0 to 10; o representing a cloudless sky, and 10 a perfectly overcast sky. In cases of a double description, the first applies from noon to midnight: the second from midnight to noon.

The principal elements for determining the general character of the weather for each month, together with remarkable meteoric phenomena, are given at the end of the "Daily Results." The phases of the Moon are given at the foot of each page.

Diurnal Inequalities of Mean Monthly Meteorological Elements, derived from the Two-hourly Indications

of the Photographic Instruments, 1862.

These results are precisely similar to those given by Mr. Johnson in 1857, excepting that as, for economy of printing, the actual Meteorographic Registers of Bi-hourly Results are not given in this volume, it was necessary to give for the Barometer and the Dry and Wet Thermometers the Mean Monthly Indications for every Two Hours. These results form Tables I, IV, and VII.

Tables II, V, and VIII give the constants arising from the solution of Bessel's Interpolation Equations, for expressing in the usual periodical formulæ the readings of the Barometer, Thermometers, &c., at any hour of the day, reckoned from noon, in terms of the mean value and the hour of the day. In the case of twelve values of a function observed at equal intervals of time, the solution is the following:

=

If B, B+ a Sin (x + A)+6 Sin (2x+B)+c Sin (3x+C)+ &c., (where is the hour expressed in degrees,) and if o, i, ii, iii,.............. x, xi, represent the values of the function corresponding to values of x, 0°, 30°, 60°,...... 300°, 330°, then we shall find that the application of the method of least squares leads to the following determination of the values of the constants a, A, b, B, c, C, &c. :

6a Sin Ao-vi+ (i-v+xi- vii) Sin 60° + (ii-iv+x-viii) Sin 30°, 6a Cos Aiii- ix + (ii − x + iv - viii) Sin 60° + (i− xi + v− vii) Sin 30°, 66 Sin Bo-iii+vi-ix + (i-ii+v-iv + vii-viii+ xi-x) Sin 30°, 6b Cos B = (i-ivii-v+ vii-x+viii-xi) Sin 60°,

6 c Sin C = o-ii + iv-vi + viii — x,

6 c Cos Ci—iii+v — vii+ix − xi.

The values of the constants being thus obtained for the barographic, thermographic, and hygrographic results, Tables III, VI, and IX, are formed by means of them, by substituting for in the general

formula the hourly values, namely, 0°, 15°, 30°, 330°, 345°, and thus the hourly values of the diurnal inequalities, or the excesses of the hourly values above the means for the day, are obtained.

Table X, giving the Mean-Monthly Elasticity of Vapour for every Two Hours of the Day, is deduced by means of Mr. Glaisher's Tables from the two-hourly values of the temperature of the air and the temperature of evaporation, and the mean values of these quantities are formulated as in the preceding instances. Table XI, giving the Mean-Monthly Values of the Pressure of Dry Air for every Two Hours, is derived from Table X, by subtracting the numbers in that Table from the corresponding numbers in Table I. It may be observed, however, that the correctness of this process has been recently disputed by Dr. Lamont.

Results for the Direction and Velocity of the Wind; Amount of Rain collected in each Month, and Distribution under different Winds; Quantity of Ozone, &c., &c.

Table XII is formed in the following manner:

Suppose the wind to have blown with a velocity V, represented in its statical effects by a pressure P, computed by the equation P

V2

200

(the velocities being those recorded in the Photographic Register, and read for every two hours for each day of every month); and let the frequency with which it has blown from any one direction making an angle a with the Meridian towards the North at any hour, be denoted by n. Then P may be considered to be the whole force of this wind during the month, and may be resolved into its components nP Cos a and nP Sin a in the direction of the Meridian and at right angles to it, being considered positive towards the North and East. The sums of the forces of all the winds for the month at this particular hour, resolved in the directions of the cardinal points will be, therefore, Σ (nP Cos a) and (P Sin a); and, if we suppose that these are the components of a single force R, making an angle with the Meridian, which would produce the same effect, (if all were blowing at the same instant of time,) we shall have

sentative of the pressure of the wind at any hour will be A Sec

Ση