We are next to imagine a sphere increasing in heat unequally from the poles to the equator. In this case, the currents will set as before, and at nearly the same altitude, but with unequal velocities in different parts of their course. The height of the barometric column at the surface will still be invariable; if as has been so far supposed, the heat be communicated to the atmosphere immediately from the sphere, and be slowly transmitted from the lower to the upper strata. But the influence of any partial and temporary source of heat, the agency of which is entirely confined to the higher regions of the atmosphere, will produce a different train of phenomena. This local increase of heat will augment disproportionately the elasticity of the superior strata, and will, therefore, disturb the regular flow of the equatorial current. A fall of the barometer wherein this disturbance takes place will be a necessary consequence of the diminished density of the atmospheric column.

The second part of Mr. Daniell's first essay is devoted to the consideration of an atmosphere of pure unmixed aqueous vapour. If the temperature of the sphere be supposed to be 32° on every part of its surface, the experiments of Mr. Dalton have shown that the elastic force of a vaporous atmosphere would at the surface be equal to 0.2 of an inch of mercury. The density of such an atmosphere would, from statical principles, decrease in a geometrical progression for equal heights. But supposing the temperature of the sphere to increase as before from the poles to the equator, it is evident that on the principle of the cryophorus, the elasticity of the whole vaporous atmosphere would be determined by that at the lowest point. Mr. Daniell supposes, therefore, that the passage of the vapour from one point to another is mechanically retarded, so as to enable it to assume the gradations due to the temperature of the subjacent part of the sphere. The direction of the currents would, in this case, be the reverse of that of a permanently elastic fluid, and they would flow from the equator to the poles, instead of from the poles to the equator. For increase of temperature augments both the density and elasticity of aqueous vapour, when in contact with water; whereas in a free atmosphere of a permanently elastic fluid, increased elasticity is always accompanied by dimínished density. At different elevations, the aqueous vapour would naturally assume the temperature due to its density. But if the heat of the higher strata be supposed to be diminished by any cause at a greater rate than is due to this natural gradation, a partial condensation must necessarily ensue.

Under the third division of Essay I. Mr. Daniell proceeds to inquire into the relations of a compound atmosphere, formed by the combination of aqueous vapour with a permanently elastic fluid. The basis of this investigation, as of the two former, is founded on the discoveries of Mr. Dalton. That philosopher (in his "New System," p. 150) was the first to reject the com

mon hypothesis of the chemical union of mixed gases, and to substitute in its room a theory better according with observed phenomena, and established by a series of new and important experiments. The leading principle of this theory is, "that the particles of one gas are not elastic or repulsive in regard to those of another gas, but only to the particles of its own kind." Hence it is inferred, that the gases which constitute our atmosphere exercise no further action upon each other than a mechanical opposition when in motion. The aqueous vapour will then be subjected to no additional pressure by commixture with a permanently elastic fluid. It will, however, be greatly modified by the temperature of the gaseous atmosphere. For example, at an elevation of 5000 feet, the density of an unmixed atmosphere (that at the surface being taken as unity), would be 0.897 of an inch, and its temperature consequently 76.5° Fahr. The temperature of an atmosphere of a permanently elastic fluid would however, at the same elevation, be only 64-4°. A mixture of the two atmospheres must then be necessarily accompanied by a condensation; for vapour of 897 density could not subsist at a temperature of 64.4°. Supposing this condensation to have taken place, and each stratum of air to possess the exact quantity of moisture due to its temperature, the two atmospheres will still be in a state of intestine motion. For the elasticity of the vapour formed at the surface of the sphere not being counterbalanced by an equivalent pressure from above, that vapour must be continually ascending into the higher regions of the atmosphere, where it will be condensed, and will give out its heat to the ambient air. A reference to our former example may serve to elucidate this general position. It appears from the calculations of Mr. Daniell, that the natural state of an atmosphere of pure aqueous vapour diffused around a sphere of the uniform temperature of 80°, would require, at the elevation of 5000 feet, vapour of the density 897. Under these circumstances, the pressure of the superior strata would exactly balance the upward tendency of the lower, and perfect rest would necessarily result. But in a mixed atmosphere, it has been already shown, that the density of vapour, at an equal elevation, would be only 636, or what is due to the temperature of 64°. Hence the pressure of this vapour will not be adequate to counteract the expansive force of the lower strata. Therefore the vapour formed at the surface will ascend into the colder regions, will be there condensed, and will impart its constituent heat to the surrounding medium. Here then is to be found the partial source of heat, to which a tacit reference has been made in the first part of the essay. The elasticity of the higher strata of the atmosphere will be augmented by this accession of temperature, and the velocity of the equatorial current will receive a disproportionate increase. To the irregularities of pressure thus produced are attributed by Mr. Daniell the fluctuations of the barometer.

We have thus endeavoured to give a concise view of a theory framed to account for the changes which are constantly taking place in the pressure of the atmosphere. It has certainly the merit of ingenuity, and, so far as we are aware, of novelty, but it rests upon the sandy foundation of assumed partial changes of temperature in the higher regions of the atmosphere, of the existence of which we have very insufficient evidence, and which, moreover, if they were by any train of reasoning rendered probable, could scarcely be considered as adequate to explain the phenomena. For to evolve so much heat as would raise the temperature of a considerable mass of air, and cause it to diffuse itself rapidly into distant regions, would require the condensation of a greater quantity of aqueous vapour than is likely to be present in any given space, and also that this condensation should not be gradual, but should take place suddenly to a very great amount. There can be no discredit, however, to any one who fails to unfold the causes of phenomena which have been acknowledged by one of the first philosophers of the present times to have hitherto baffled all attempts to reduce them to fixed principles. The data for a sound and stable theory are, it appears to us, still wanting, and must be supplied chiefly by a very extensive series of simultaneous observations on the state of the barometer, in various and distant parts of the world.

*

We may remark, by the way, an error, as it seems to us, into which not only Mr. Daniell (p. 8), but Mr. Leslie, has fallen, viz. " that the particles of air in passing over the surface of the globe do not for a moment cease to gravitate, and that no horizontal movement of them will produce the slightest derangement in a perpendicular direction." Now it is well known that any body, to which a projectile motion of five miles per second has been imparted, would revolve around the earth like a planet, and would cease to exert any pressure on its surface. Any less velocity must produce a proportional decrease of weight in the particles of air, which is known to move at the rate of from 60 to 100 miles per hour.

480

We venture also to suggest, with submission, that the third table in Part I. is founded on an erroneous principle. In calculating the influence of a decreasing temperature on the weight of the atmosphere at different heights, Mr. Daniell has deducted 1 of the length of the mercurial column for each degree of depression due to the elevation. Now it appears to us, that a mean ought to have been taken between the temperature at the base, and that at the summit of the atmospheric column. For example, the weight of a column of air of 5000 feet, supposed of an uniform temperature of 32°, and decreasing in density from the surface upwards, according to statical laws, is equal to

* M. Biot,

5.203 inches of mercury (see Table I, p. 13, of Mr. Daniell's work). The barometer, therefore, which, at the surface of the supposed sphere, stands at 30 inches, will, at this elevation, indicate 24-797. But if the temperature of this aerial column gradually decrease from 32°, till, at the height of 5000 feet, it becomes 14.8°, it is required to determine the change which this variation will produce in the height of the mercurial column at the above elevation. The question seems to us to reduce itself to a simple comparison between the weight of a column of air 5000 feet high, of the temperature 32° Fahr. and that of an equal column of the temperature 23.4°, which is the mean of the temperature at the base and that at the summit. Now air by being reduced 1° Fahr. contracts in bulk of the volume which it would occupy at 32°; consequently a reduction of temperature equal to 8.6 (32′′ 23"-4) will be accompanied by a decrease of volume equivalent to of its former bulk.

8.6

480

8.6

480

1 480

The

vacuous space which would be left by such a contraction must be immediately filled up by air from above. Hence the mercurial column at 5000 feet must, by falling, indicate this transference of air from the superior to the lower strata, and this fall will be equal to of 5-203093. At the elevation of 5000 feet then, the height of the barometrical column will be equal to 30520309324-704, instead of 23.949, the number given by Mr. Daniell. The same result will be obtained by means of a formula derived algebraically from one originally given by Sir G. Shuckburgh.* Let H denote the height of the mercurial column at the surface of the earth, y that at a given elevation p (in the present instance 5000 feet), and b the number of feet of air of the given temperature (23′′-4), equal to 1-10th inch mercury.

600 b P

600 b + p

Then y = values of b and p, the former of which is obtained from a table

× H. Substituting in this formula the

600 × 85.044 5000 given by Sir G. Shuckburgh, we have y= × 30 600 x 85-044 + 5000 24.64. The small difference between this result and the former one may be attributed to Sir G. Shuckburgh's having estimated the expansion of air for each degree Fahr. at 435

1

instead of of its original bulk.

480

Our limits will not permit us to enter at any length into the account of Mr. Daniell's hygrometer, which is fully described in his work, and also in the Quarterly Journal, Nos. 11 and 25 We consider it as an elegant instrument, and are satisfied by

• Dalton's Meteorological Essays, p. 82.

trial of it that it is adequate to its object, that of ascertaining quickly and correctly the temperature at which dew begins to be deposited. But we are not aware that in accomplishing this, it has any great advantage over the method of Le Roi, which is recommended by the extreme simplicity of the apparatus required. This consists of nothing more than a thermometer and a glass tumbler filled with water, the temperature of which is lowered by gradually adding ice (nitre or sal-ammoniac would answer the same end) till dew begins to appear on the outer surface of the vessel. Noting this point, whether obtained by Le Roi's or Mr. Daniell's method, we then find, from Mr. Dalton's table, the force of vapour at that temperature; and from the proportion which this force forms of the whole pressure of the atmosphere at the time, we at once arrive at the absolute quantity of vapour in a given space. We regard the indications of this simple process as much more satisfactory than those of Mr. Leslie's hygrometer, because, to deduce from the latter the real proportion of vapour in air, requires a much more complex calculation, of which some of the data, or of the steps, may possibly be erroneous.

The remaining essays of Mr. Daniell we are obliged to pass over without any notice. Indeed being chiefly composed of details of facts, they are not from their nature susceptible of abridgment. They are important, however, to those who are practically engaged in making or recording meteorological observations, and to all such persons, as well as to those who are interested in the theory of atmospheric phenomena, we can safely recommend the work as containing an ample fund of valuable information.

Z.

The Elements of Pharmacy, and the Chemical History of the Materia Medica, &c. By Samuel Frederick Gray, Lecturer on the Materia Medica, Botany, and Pharmaceutic Chemistry.

It is impossible to deny that this work is calculated to convey a considerable portion of information; but it inust at the same time be admitted, that much of it will be of little use to the student. The arrangement (if indeed arrangement it can be called) is peculiar, and while some subjects are treated of with extreme brevity, there are others which are extended much beyond the requisite limits; thus weights, measures, and balances, occupy about 20 pages, furnaces 33, and the theory of chemistry 34. The properties of atmospheric air and water are then detailed; lead, copper, tin, and some other metals, are next treated of in six pages; and we are then surprised with an account of the "alchemy of the Greek clergy," "the introduction of alchemy into the west," and the "original theory of transmutation; "these disquisitions occupy about seven pages.