The Lehigh Zinc Co., since taking possession of the factory, have maintained the high character of its product, and the zincrolling mill which they have added to it turns out sheet zinc of the very choicest quality, such as could only be made from spelter of the highest grade.

This excellence results mainly of course from the unusual purity of the ores of Saucon Valley; the silicate particularly, which has always been the predominant ore there, being notably freer from impurities, not only than most ores from other localities, but also than the carbonate which abounded in the earlier history of those mines, or the sulphide which appears more and more as the depth of the workings increase.

At one time, in order to produce a certain quantity of superior zinc for chemical uses, I selected a quantity of the cleanest silicate of zinc, calcined it in a new roasting furnace, rejected, after roasting, all pieces which from discoloration or otherwise appeared suspicious, ground it in a clean mill, and then distilled it in a furnace which had never before been used, rejecting all the first and last products of distillation from each charge in each retort. The zinc thus made, amounting to several tons, went mostly to various chemists, and is doubtless all consumed except a few ingots which I still retain. It was, however, no purer than that examined by Drs. Eliot and Storer, nor is this surprising, since the latter was made in a new furnace from silicate of zinc which had been weathered for a long time, and was thus freed from the intermingled clay which otherwise might have yielded some impurities.

The establishment whose origin is thus sketched is still in successful operation, and though its activity is now limited by reason of an insufficient supply of ore, there is good reason to believe that this difficulty will be overcome when the magnificent pumping apparatus now being erected by the Lehigh Zinc Company shall enable them to mine at greater depths.

The entire spelter and sheet zinc manufacture of the United States, now a large and growing industry, may fairly be said to have sprung from this factory, for not only was it the pioneer in point of time by at least two years, but I believe that neither of the others succeeded until it availed itself of the services of men procured from this establishment.

ART. XXV.—The Daily Motion of a Brick Tower, caused by Solar Heat; by Prof. C. G. ROCKWOOD, Ph.D., Bowdoin College.

THE observations which form the subject of the following discussion were made in the spring of 1866. Some recent notices of an attempted investigation of the effect of solar heat upon the dome of the Capitol at Washington have induced the publication of the present note; which, though not a perfectly satisfactory solution of the question, it is hoped may still possess some interest, as being, so far as I am aware, the most accurate investigation of the subject that has been made.

The observations were conducted in the south tower of the Sheffield Scientific School in New Haven, Conn. This tower was built during the winter of 1865, but was still unfinished in April, 1866, when these observations were commenced, although all the brickwork was completed. The stuccoing of the outside was not finished until June 1, and during the whole time occupied by the investigation, the presence of the workmen, with scaffoldings, etc., although not vitiating the truth of the results obtained, was a hindrance to that perfect success which might be expected from a repetition of the experiments under the more favorable conditions of a completed tower and more delicate instruments.

The structure in question is a square brick tower, stuccoed on the outside, the exterior surface being much broken up by recessed windows and various architectural adornments. The general plan of the building precluded the use of solid masonry in the tower, which would have been desirable for any structure designed, as was this, to support astronomical instruments. The walls were, however, made unusually heavy, and in order to have as firm a base as possible for the telescope, the upper story was arched with brick, forming a solid and pretty firm brick floor, upon which now rests the stone pier of an equatorial.

The whole tower is surmounted by a revolving wooden turret. The story below the observatory, and immediately beneath the brick arches, is occupied by the works of the tower clock.

The dimensions of the tower are as follows:

Side of the square at ground,.

top of brickwork, . . . Thickness of walls at first story,..

top of brickwork, . - - where the arches spring,...

Height to top of turret,

brickwork,

floor of observatory room,-.

16 feet.

The tower is connected by its north side with the main building for an altitude of about 45 feet. Projecting from the south side of the main building, it has its north wall in a line with, and forming part of, the south wall of the building. The whole edifice stands in a position inclined to the meridian, the sides of the building, and consequently those of the tower, having the direction N. 28° 30' E.

Previous observations elsewhere (at Bunker Hill Monument) had led to the apprehension that such a tower, besides being subject to tremors communicated from the ground, would have a definite and somewhat regular daily motion, dependent upon the influence of the sun's heat in expanding the materials of which it was composed. The object for which this investigation was undertaken was to ascertain whether this motion would affect the use of the telescope or not.

In order to investigate the motion in the present case, two levels were placed at right angles with each other, upon a flat stone embedded in the brick floor of the observatory room (to avoid any undetermined changes which might affect the woodwork), and their indications were recorded from time to time. Any tipping or motion of the tower would of course change the plane of this floor by the same amount, and would be shown by a corresponding motion of the bubble of one or both of the levels. The levels used were:

I. The striding level of a small transit instrument, the property of Prof. C. S. Lyman. This level, supported by its iron stand, was placed parallel to the front of the tower, and therefore, approximately east and west. It was read by a scale graduated to hundredths of an inch. Examination with a level-trier gave 1 inch 39" 37 as the value of its divisions.

II. An unmounted Ertel bubble, which rested directly upon the stone, being placed parallel to the sides of the tower, and so approximately north and south. It was read by a scale of tenths of an inch, graduated upon the glass, and reading by estimation to hundredths. Prof. Lyman had previously determined the value of the divisions of this level at 1 inch=51"-7, which was adopted in reducing the observations.

The levels were recorded six times during the day, at intervals of about 2 hours; the first record being between 7 and 8 A. M., and the last between 10 and 11 P. M. The series ex

tended, with some omissions, from April 24th to July 7th; but some of the records being afterward rejected as unreliable, the final results were obtained from the discussion of observations upon 59 days, included between April 24th and July 2d, 1866. It will be noticed that about one-half of this period was before the stuccoing of the tower was completed; and during about two weeks, from May 15th to June 1st, the tower was partially shaded from the sun by the scaffolding necessary to this work.

A third level placed with the others upon the stone floor, and also the levels of a zenith telescope, which was temporarily mounted on a brick pier in the room, were recorded during a part of this time, but as their results were not employed in the discussion, it is not necessary to notice them farther than to say, that in general they confirmed the indications of the principal levels.

Let us now examine briefly what would be the probable motion of a tower thus situated, and then compare this theoret ical result with that given by the recorded level readings.

First: suppose an isolated symmetrical tower of homogeneous material, situated at the equator of the earth, and the sun at the equinox. The diurnal circle of the sun then passes through the east and west points of the horizon and the zenith of the tower. In the morning the heat of the sun's rays would expand the east side of the tower, and cause it to lean toward the west. As the sun rose toward the zenith, and warmed equally all parts of the tower, it would gradually return to its mean position. In the afternoon, as the east side lost its heat by radiation and the west side was warmed by the declining sun, it would lean toward the east; and during the night would return again to the mean position. Thus the motion would be back and forth over a straight east and west line.

Again, suppose the same tower situated at the pole. Then, since the sun's rays strike it during the whole twenty-four hours, and always at the same angle, the tower, leaning always from the sun and always to the same amount, would follow the sun in its diurnal revolution, its top describing a circle about its normal position.

At any station intermediate between the equator and the pole, the figure described by the tower would be neither a straight line nor a circle, but between the two, i. e., an ellipse, whose excentricity diminishes as the latitude of the place

increases.

In the case under discussion, the tower is in N. lat. 41° 19', and the sun was near the summer solstice. The sun therefore rose about 30° north of the east point, culminated south of the zenith, and set north of west.

The inclination of the tower, being opposite the sun, would be, in the morning southwest and west, at noon north, at evening east and southeast, and during the night it might be supposed to return in a straight line to its mean position. Since the sun's rays strike the perpendicular sides of the tower more obliquely at noon than at morning and evening, the northward inclination would be less than the westward or eastward, and the curve described would resemble an ellipse, with its minor axis in the meridian, and probably somewhat flattened on the

south.

This tower also is not symmetrical nor isolated. It may be s considered as a right parallelopiped, standing with its sides inclined to the meridian, and for half its height joined by one side to a large building, which, so far as any effect of the solar heat is concerned, may be regarded as fixed. One side of the tower being thus firmly held by its connection with the building, any motion toward or from the building would be simply checked or retarded without change in direction; while to any motion at right angles to this would be added another element, a motion in azimuth or a twisting of the tower about a vertical line. A scale was prepared in a distant steeple, and the zenith teles- a cope already mentioned was directed upon it, with a view to determine any such azimuthal change. The observations with t this arrangement showed a probable daily change of 8" or 10′′ in azimuth, but the instability of the telescope and other causes so far impaired their value, that it was not thought best to incorporate them in this discussion.

The recorded observations give a series of level-readings at certain hours upon 59 days. The most obvious method of combining them is to take the mean of the readings for all the days, as representing the probable motion of the tower for any one day. Thus combined, the means are as follows, the levels being designated by the numerals I and II, and the level readings having been reduced to seconds.

If now we let these numbers be the abscissas and ordinates of a plane curve, referred to rectangular axes in the direction of the levels, this curve will be the figure described daily by the normal to the plane of the levels or by any vertical line of the tower. It should also be borne in mind that a south level reading indicates a north inclination of the tower, and vice versa; therefore, to represent the actual motion of the tower the signs of the above means have been changed throughout in plotting the curve. The curve thus obtained is the one marked A in the figure, the points given by the observations being marked by the numerals 1, 2, 3, etc., in order, beginning with the morning observation. It is seen to be an imperfect ellipse, with a major axis of about 12" and a minor axis of about 5", and the minor axis coincides nearly with the meridian. It thus corresponds tolerably with what had been anticipated. But the minor axis of the ellipse is not exactly in the meridian, and the curve is somewhat flattened on the southwest and northeast. Apparently the connection of the tower with the main building has checked the north and south motion, and so shortened