"It is not always an easy matter to meet with a current of water sufficient to move the bellows required in smelting on an hearth furnace; and to carry the ore from the mine where it is dug to a considerable distance to be smelted is attended with great expense: this expense is saved by smelting in the cupola furnace, which, not requiring the use of bellows, may be constructed any where; wood is very scarce in every mining country in England, and the pit coals cost ten or twelve shillings a ton in Derbyshire, yet they can smelt a definite quantity of ore in the cupola at a far less expense, by means of pit coal than of wood. The flame which plays upon the surface of the ore, and smelts it in a cupola furnace, is not driven against it with much violence; by this means small particles of ore called belland may be smelted in a cupola furnace with great convenience, which would be driven away if exposed to the fierce blast of a pair of bellows in a hearth furnace. These are some of the advantages attending the use of a cupola in preference to a hearth furnace, and to these may be added, one superior to all the rest, the preservation of the workmen's lives: the noxious particles of the lead are carried up the chimney in a cupola, whilst they are driven in the face of the hearth smelter at every blast of the bellows.

"They generally put into the cupola furnace a ton of ore, previously broken sinall, and properly dressed, at one time; this they call a charge: if the ore is very poor in lead, they put in somewhat more, and they work off three charges of ore in every twenty-four hours. In about six hours from the time of charging, the ore becomes as fluid as milk.

"Before the ore becomes fluid, and even whilst it continues in a state of fusion, a considerable portion of its weight is carried off through the chimney; what remains in the furnace consists of two different substances, of the lead, for the obtaining of which the process was commenced, and of the slag or scoria. The proportion between these parts is not always the same, even in the same kind of ore it depending much upon the management of the fire. The lead, being heavier than the slag, sinks through it as it is formed, and settles into the concavity of the bottom of the furnace. The pure slag, according to the idea here given, is that part of the ore of lead which is neither driven off by the heat of the furnace, nor changed into lead. In order to obtain the lead free from the slag which swims over it, the smelters usually throw in about a bushel of lime; not, as is usually supposed, in order to contribute towards the more perfect fusion of the ore, but to dry up the slag A a 4 which

which floats on the surface of the lead, and which, being as liquid as lead, might otherwise flow out along with it. The slag being thus thickened by an admixture of lime, is raked up towards the sides of the furnace, and the lead is left at the bottom. There is a hole in one of the sides of the furnace, which is properly stopped during the smelting of the ore; when the slag is raked off, this hole is opened, and being situated lower than the lead in the furnace, the lead gushes through it into an iron pot placed contiguous to the side of the furnace; from this pot it is laded into moulds, each containing what they call a pig of lead: the pigs, when cold, being ordinarily stamped with the maker's name, are sold under the name of ore lead. After the lead has all flowed out of the furnace, they stop up the tap-hole, and drawing down the slag and lime into the middle of the furnace, they raise the fire till the mixture of slag and lime, which they term simple slag, is rendered very liquid; upon this liquid mass they throw another quantity of lime, to dry it up, as in the former part of the process. This second mixture of slag and lime is then raked out of the furnace, and the small portion of lead separated from the fusion of the first generally to the amount of twenty or thirty pounds, being let out of the furnace, a new charge of ore is put in, and the operation recommenced. In order to spare the lime and the expense of fuel attending the fluxing of the mixture of le and slag, they have in some furnaces lately contrived a hole, through which they suffer the main part of the liquid slag to flow out, before they tap the furnace for the lead; upon the little remaining slag they throw a small portion of lime, and draw the mixture out of the furnace without smelting it. This kind of furnace they have nick-named a maccaroni."

LXVIII. On the Error discovered in the Nautical Almanac. By Mr. FIRMINGER, late Assistant at the Royal Observatory, Greenwich *.

To Mr. Tilloch.

SIR, In your journal for the last month, your readers will IN N

doubtless notice, with much surprise, an account of an error in the Nautical Almanac for the year 1812, which is therein said to have been recently discovered by Dr. Kelly. To remove, however, from the minds of such readers and others, whom this circumstance may happen to interest, the mis

Teacher of Astronomy, Mathematics, and the various branches of Natural Philosophy.

take

take into which that statement may tend to lead them, as well as the illiberal insinuation which it tends to throw upon the French astronomers and mathematicians, by challenging them with an unjust adoption of the results derived from the Nautical Almanac, into their Connoissance des Tems, under the mask of pretended originality, has given occasion to the production of this article; a circumstance which, from a long residence at the Royal Observatory, and of course a more familiar acquaintance with the habits of the late Dr. Maskelyne, I may be conceived to be better able to answer than others less acquainted with that justly celebrated

man.

Dr. Maskelyne says, in his preface to the Nautical Almanac of the year above alluded to, that he has taken the mean obliquity of the ecliptic for the beginning of the year at 23° 27′ 43′′,8, which he reduces to the apparent obliquity by applying the equation arising from the precession of the equinoxes, combined with a diminution of half a second a year from a change of this quantity in the plane of the obliquity itself, and an equation depending on the place of the moon's node arising from her action on the spheroidal figure of the earth these two equations are, as Dr. Maskelyne states, contained in two tables which were published at the same time with the first volume of the Greenwich Observations. The first equation amounts to -0,5, and the second to -9′′,0; their sum is -9,5 which applied to 23° 27′ 43′′,3, the mean obliquity above mentioned, gives the apparent obliquity of the ecliptic 23° 27′ 33′′,8 at the beginning of 1802.

By comparing this deduction with that given opposite the first page of the Nautical Almanac, it will be found exactly the same.

The obliquity of the ecliptic at the beginning of the preceding year, viz. 1811, Dr. Maskelyne states in the preface of the Nautical Almanac to be 23° 27′ 51′′9, and at the beginning of the year 1813 he takes it to be 23° 27′ 51′′,3; so that the mean obliquity of the ecliptic at the beginning of 1812, it should appear from these two statements, ought to be 23° 27′ 51′′,6, instead of 23° 27′ 43′′3 as mentioned in the Almanac for that year. This apparent discrepancy making a difference of 8",3, may seem to give sanction to the account contained in the Philosophical Magazine of last month, that the difference in question might have probably originated in mistake. Dr. Maskelyne was however by far too careful a man to suffer such a circumstance to have escaped his notice; and the frequent use of the obliquity of the ecliptic in the calculations that were constantly carrying

on

on at the Royal Observatory, rendering the accuracy of that datum a circumstance of the greatest importance, was a means which constantly drew the doctor's attention to that subject. He had been accustomed to settle the mean obliquity, as well as its secular variation, from observations made on the sun's zenith distances taken for ten days before and after the summer solstice, and reduced to the time of that solstice, instead of taking a mean of the reductions derived from the two solstices. This he did in consequence of the discrepancy in the results derived from each solstice; and as he was unable to say what produced this difference, and knowing (at that time) of no cause but the uncertainty of refraction, he was induced to give a preference to the deductions derived from the sun's zenith distance at the summer solstice. Here, however, another difficulty presented itself to him:-By a comparison of the deductions thus obtained with the same derived from observations made during the life of Dr. Bradley, the secular variation of the obliquity of the ecliptic came out much less than the secular variation derived from the late observations, or those in the time of Dr. Bradley with more ancient ones, in which a much longer period had been embraced; so that the variation of the obliquity of the ecliptic must from such determination appear to be decreasing less than formerly, for which no cause could be assigned. This led Dr. Maskelyne to consider whether he was right in giving more weight to the deductions made from the observations at the summer solstice than those at the winter solstice; and finding the deductions from the latter agreeing better when compared with those made in the time of Dr. Bradley, in giving the secular variation nearer to what arises from either of those observations when compared with ancient observations, induced him at last to adopt them in preference to the deductions derived from observations about the summer solstice.

It will be recollected that I have already said that Dr. Maskelyne knew of no cause to give preference to either; and it was a circunstance of anxiety to him to see so great a difference between the results at the two solstices as the Greenwich quadrant gave, without being able to assign a reason. Knowing the care taken in making the Greenwich observations, and their superior accuracy over those made on the continent, prevented him from relinquishing his deductions for an adoption of others which he could not bring his mind to believe were so much to be depended on. The suspense which this state of things induced, was not however to be of long continuance. A series of observations made by Mr. Pond, our present astronomer royal,

on

on the declinations of the principal fixed stars, taken with an excellent meridian circular instrument made by Mr. Troughton, and his comparison of those observations with a like series of observations made with the mural quadrant at Greenwich, pointed out to Dr. Maskelyne a circumstance of which till then he had never even admitted a conjecture, viz. that in the course of the number of years in which the mural quadrant at Greenwich had been suspended, it must have changed its figure. Dr. Maskelyne viewed Mr. Pond's observations at first with a very cautious eye; but having satisfied himself that they were taken with the greatest care and ability, he availed himself of the deductions drawn from them, and occasionally made use of their results, as corrections applied to observations made with the Greenwich quadrant. He brought back the obliquity of the ecliptic to nearly the same quantity he had before adopted; but whether he ever intended to give a correction to the Nautical Almanac of 1812, or not, I am unable to say. It is however likely he did not think the difference of sufficient importance to require any thing more than a note, merely stating what the obliquity of the ecliptic should be: indeed it is uncertain even to the last, whether Dr. Maskelyne was altogether satisfied on this point; for Mr. Groombridge told me a long time ago, that he mentioned this circumstance to Dr. Maskelyne, who in reply said he should see by and by; that he was not sure it was wrong, but if it turned out so, he should correct it. It is worthy here of remark also, that Dr. Maskelyne never readily received notice of any inaccuracies in his deductions. Inaccuracies had been often pointed out to him by various persons, which, upon examination, were generally found to originate in a want of knowledge on their part of the true principles upon which the calculations were founded. A seeming indifference therefore in the conduct of Dr. Maskelyne was generally remarked, and complained of by persons who gave him such information, in his attention to their statements.

Having already observed that the mean obliquity of the ecliptic at the beginning of the year 1812 was given by Dr. Maskelyne 23° 27′ 43′′,3, and that the apparent obliquity was 23° 27′ 33′′,8, differing by 9′′,5 from the mean obliquity; it appears from the statement in the Philosophical Magazine, that this difference has led to a conclusion, that the mean obliquity had been twice equated; and it is curious enough that the quantity of mean obliquity as assigned by Dr. Maskelyne should have been nearly this quantity different

from