ing, it is true, at different times. Now if it cannot be proved that this electricity is dependent on extraneous circumstances for its production, we are fully justified in assuming that an electrical state is the normal condition of the atmosphere. Thus the earth, as a whole, must always possess the same amount of electricity in activity; if deficient in one part, it would be superabundant in another, and no ultimate loss of even this most subtle of all things which our senses have made manifest, can take place in a different manner from that in which the air or water which exists all over the world can occur. These are constantly entering into new combinations, which, in the lapse of ages, might diminish their quantity in the form in which they now occur; so the number of compositions and decompositions which are perpetually occurring, and which undoubtedly involve the consumption (so to speak) of electricity, placing it in a form not recognizable by previous means, might also, in the course of immense periods, have a material effect on that existing in a free state, and thus greatly alter the phenomena depending on its agency. But in the course of short periods, such changes would be inappreciable, and therefore on these grounds there can be no cogent reasons urged against the supposition of the air's electricity being a primitive and essential component of our globe.

The earth, then, being an immense sphere having the property of conductability, at least on its surface, must necessarily, as mathematicians can readily prove, appropriate all free electricity contained in it to its surface, and, if surrounded by any non-conducting medium, must remain there; but if not thus surrounded, will quit the surface entirely until it attains a position of equilibrium. From this cause, the tendency of any electricity existing on the surface of the earth, is, by the law of distribution, towards the higher part of the atmosphere, until it attains an equilibrium, the place of which it is, of course, impossible to assign. In this there is nothing in the slightest degree hypothetical; for experiments and reasoning, as well as the fact of the air being found more highly charged in proportion to the height, concur in enforcing this as true. Whether we admit the electricity to be produced on the

earth, or to exist naturally associated with it, this must be granted,-that its place of equilibrium is without the surface. The atmosphere, when in its ordinary state, is not a perfect non-conductor, and at times its conductability is considerably increased by the mixture of condensed vapour. This variable change, as respects the power of conduction, is the cause of those tremendous disturbances which are exhibited in the form of thunder-storms and their constant phenomena. Were the conducting power of the air uniform, then would the distribution of electricity be constantly the same, and the lightning's vivid flash and the thunder's terrific roar be entirely unknown.

As a complete exposition of the consequences of our view of the subject would occupy much space, for the present we will confine ourselves to the rationale of a thunder-storm, and, if acceptable, we will in future papers discuss the manner of treating other adscititious electrical phenomena.

A thunder-storm is usually the result of an influx of a cold into a warm current of air. The warm air is at such times much more charged with vapour than the cold, and as the quantity of vapour adequate to the saturation of the mixture at the resulting temperature is less than the sum of the quantities subsisting in each separately, a part must be precipitated, and thus form a cloud, the density of which depends on the quantity of vapour contained in the mixed air. In this manner there is formed a conducting body in the midst of a mass of air, the electricity of which is more intense at its higher extremity than at its lower. The cloud, as a conducting body, will determine the distribution of the electricity, thus giving liberty of motion to its surface, while the presence of the earth will cause an accumulation at the lower end by the laws of induction. Now, a cloud being but imperfectly conductible as well as imperfectly opaque, the distribution takes place, not insensibly and invisibly, as in a perfect conductor, but produces a flash and sound perfectly analogous to what occurs, when in our experiments we make a film of water trace on glass a part of the circuit of discharge of a powerfully excited body. Rapidity of condensation is indispensable to this discharge. Were it

structed from the "index" to the square as follows: of :- divide the "index" the square into two equal and similar parts; apply sights and bring the parts into such a position as, that if one of them were to revolve through 180° round their common axis, (the other remaining fixed,) both parts with their respective sights would coincide.

Such a contrivance as this alhidada would obviously be desirable when the circular form of theodolite was employed, but when the semicircular one was used (see the quotation in the first paragraph of this letter) would not the simple form of "index" be adopted? In the latter case there would be no obstacle to the eye being placed near the centre of the semicircle, and the reason for varying the form of the simple "index with sightes" of the " square geometricall" would cease. So that the semicircular theodolite would appear to be complete without an alhidada. Does this render it necessary for us to seek some other word than alhidada as the origin of the name given to the instrument mentioned in the title of this paper?

In defining the theodolite as "but a circle divided, or a semicircle parted," combined with the fact of not mentioning the "index" till after the occurrence of two full stops, it would, perhaps, seem that the writer laid stress upon the graduation of the instrument. Did I not attach a great and deserved weight and authority to the remarks of Professor de Morgan* upon the origin of the name theodolite, I confess I should be disposed to refer it to the graduation of the instrument and to connect it with the word ὀδελός, (όβελος, ὀβελίζω.)

I am, Sir, yours, &c.,
JAMES COCKLE.

Devereux-court, August 7, 1846.

GREENER AND STAITE'S ELECTRIC LAMP. [Patent dated February 7, 1846; Specification enrolled August 7, 1846.]

We have here another attempt to realize that favourite speculation of the day -a lamp which shall burn without air or oxygen, and be fed solely by that mysterious agent which we call electricity. Not long ago we gave a plan for the purpose, of American origin, patented in England by a Mr. King, and felt obliged

* Phil. Mag. for April, 1846.

to speak of it in terms which have not been gainsaid by anything we have since heard. The present has a more promising aspect. It shows, at the least, decided originality. We shall give the patentees' description in their own words:

"The nature of our invention and the manner in which the same is performed may be stated generally as consisting in effecting the illumination of public and private buildings, streets, squares, and other public places, by means of solid prisms or cylinders of carbon, enclosed in air-tight vessels of glass or some other transparent substance, and ignited or rendered luminous by currents of voltaic or magnetic electricity, such carbon being previously freed from the impurities with which it is ordinarily combined, and divided on the surface thereof into numerous acute points-or by means of rods or strips of platinum, or some other difficultly fusible metal enclosed in transparent airtight vessels, and ignited or rendered luminous by currents of voltaic or magnetic electricity as aforesaid, such metal being also divided on the surface thereof into numerous acute points-or by means of hollow cylinders of carbon, with surfaces either plain or acuminated, partially inserted within, and placed in perfect contact with hollow cones of platinum, either plain or acuminated, the whole being enclosed in transparent air-tight vessels as aforesaid, and the carbon being ignited or rendered luminous by voltaic or magnetic electricity, also as aforesaid-or by a combination or combinations of the whole of the said means or methods, or of any two thereof, or of any of the parts thereof espectively. Both carbon and platinum have been before employed as media for the development and exhibition of electric light, but carbon, even in the purest states in which it has been hitherto obtainable when ignited, or rendered luminous in an airtight glass vessel by means of electric currents, has been found to give out various extraneous matters which interfere with the continuity of the light, and, being precipitated on the inside of the glass vessel, obscure and darken the same, and plain surfaced platinum, when substituted for the carbon in the air-tight vessel, has never yielded more than a comparatively very feeble light. Now to obtain for the purpose of our invention a carbon absolutely pure, or at least more nearly approaching absolute purity than any heretofore known, we proceed as follows: We take a quantity of what is known in commerce under the denomination of lamp black, or of charcoal reduced to a state of powder, or of coke, also reduced to a state of powder, which has been

purified by the application of electricity from sulphur and other metallic mixtures, according to the process for which Jabez Church, of Colchester, has obtained letters, patent of date the 20th December, 1845,* and digest the same in dilute nitro-muriatic, or other acid. We then strain the lamp black, or charcoal, or coke, from the acid, and wash it several times with water; we next wash it in some weak alkaline solution, or carbonated alkaline solution, and, finally, we wash it with distilled water until no traces of impurity are perceptible. The carbon thus highly purified, we next bring into a state of great dryness, and then convert it into solid prisms, or into cylinders, both solid and hollow, by means of a hydraulic, or screw fly press (in the same way as the well-known porcelain buttons are formed). And sometimes the better to secure perfect dryness and homogeneousness, we expose the said prisms, or cylinders, to intense heat in a furnace for about twenty-four hours. To give the solid carbon prisms, or cylinders, the numerous acute points on their surfaces before-mentioned, they are cast in suitably-indented discs, or the points are afterwards cut out in them by means of a fine saw. By opposing two such acuminated surfaces, the one to the other, we secure this advantage, that should the electric current cease between any two points, (as happens frequently in the ordinary mode of igniting carbon by electricity, when there are only two points in opposition,) the currents will be kept up by the remaining points, and the light be so maintained, without any material interruption or abatement. The same purpose may, in some cases, be effected by substituting for bodies with such acuminated surfaces a multitude of thin strips of charcoal separated from each other by platinum foil, and bound together by platinum wire. Instead also of employing either lamp black, or charcoal, or coke, we sometimes substitute plumbago, or black lead, prepared, formed, and used in the same way; but the sort of carbon which we prefer before all others, is that prepared in the manner before-mentioned, from coke, purified by Church's patent process. When we use platinum, or any other difficultly fusible metal, as the illuminating medium, we multiply, as much as we conveniently can, the number of separate points on its surface, and with this view, we either perforate a solid sheet of the metal with a multitude of minute holes, or we take it in the form of wire and weave it into a gauze of sufficient thickness to resist the heat produced by the electric current without fusing. When we employ the combination before

For the specification of this patent see Mech. Mag. of last week, No. 1200,

161

mentioned of hollow cylinders of carbon with hollow cones of platinum, we arrange them as follows. Supposing a voltaic battery of any of the most approved forms to be employed, we connect each of the two conducting wires, namely, the positive and negative, with a hollow cone of platinum, either plain or acuminated, and place the cones with their bases facing, but at a little distance from one another; we then connect the two cones by placing between them a hollow cylinder of carbon, either plain or acuminated, and insert it so far into the cones at the two ends, as to be in perfect contact therewith all round. The relative diameters should be such as to render it necessary to introduce the cylinder but a small way into the cones. Such cones and

cylinders may be used either in single sets, or in any number of sets, which the battery or batteries employed may be adequate to supply. The sets, too, may be arranged in any order, and they may be of any variety or varieties of size best calculated to produce an equable, well sustained, and brilliant light. When the carbon, or platinum, or other difficultly fusible metal, (in whatever form employed,) is placed within the air-tight vessel of glass, or other transparent substance, it may be either kept stationary, or made to rotate on its axis. We think it unnecessary to describe, either how the air-tight vessel which is to enclose the illuminating bodies should be mounted, or how the electric currents are to be conveyed to them from the voltaic battery, or magnetic machine employed, because there are many well-known modes of doing so, and we do not claim to have invented anything new in these respects. We desire it farther to be understood, that we lay no claim to the employment of carbon, or platinum, or other like difficultly fusible metal enclosed within airtight vessels as a medium for the development and exhibition of electric light, the suitableness of both bodies for the purpose when so enclosed (subject to the exceptions aforesaid) being long well known. But we declare, that what we do claim as of our invention, is-First, the employment as a medium for the development and exhibition of electric light of carbon purified and prepared as before described. Second, The employment for the said purpose of carbon and platinum, or any other like difficultly fusible metal in solid prisms, or cylinders, divided on the surface thereof into numerous acute points as before described. And, Third, the employment for the said purpose of hollow cylinders of carbon, either plain or acuminated in combination with hollow cones of platinum, or other like difficultly fusible metal, either plain or acuminated as before said."

Then, since the surface is a plane, the opposite sides a,b,, a,b, of the quadrilateral a,a,b,b, will meet in the trace of the plane, abad,c, on the horizontal plane, in a point g.

Whence if a,a,b,b, be rabatted upon the horizontal plane, the point g will still retain its position, and hence may be found by working wholly on the horizontal plane.

In the same manner by the rabatement of cc dd, on the horizontal plane, and producing cd, cada to meet in h, this

point will also be in the trace of the surface-plane.

Two points, g, h, therefore, being found in the trace of the surface-plane, that trace itself is found.

Again draw c, perpendicular to gh; let Aa, meet c,d, (rabatted on the plan) in m; make c,k, parallel to gk, and equal to c,m; and finally join kl. The angle c,lk is the declivity of the plane.

For c,m is the height of the point in which the perpendicular from c, meets