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casting. In this is a shallow circular groove four inches and a half wide by three-quarters of an inch deep, and of the curvature due to a radius of four inches, in which three iron balls of six inches and a quarter diameter are placed, for the dome to run upon. The dome, in form, is almost a hemisphere, and is constructed of fir-wood in several thicknesses each piece about six inches deep by eight inches and a half in width-the inner diameter of the dome being fifteen feet four inches. To the underside of the aforesaid curb is secured a ring of cast iron with a circular channel, being the counter-part of that just mentioned. From the upper side of this curb spring sixteen curved rafters of cast iron, the section of which is that of a, and the upper ends of these meet against a cast-iron ring of about three feet diameter: fifteen other intermediate rafters of similar section, but shorter, abut at their upper ends against crosspieces connecting the fifteen main rafters, the sixteenth being left out to form a clear opening for view. The rafters are filled in between with two inches and a half battens, jointed straight, rabbeted in their length, and the ends meeting at the back of the rafters, where they are firmly screwed upon the back ribs of the same. The inside of each piece of wood being flat, the whole interior is, of course, polygonal; but the outside is adzed off to the proper curve, which is covered with thin copper sheathing. The opening for observation is continued to the upper ring, and is equal to one-sixteenth of the circumference the shutter moves on a pin at the apex of the dome as a centre, and it is carried at the bottom by rollers on a bar fixed to the wood curb; there is also a toothed rack and pinion with a handle inside the dome, to give it motion. The shutter itself is in one piece of hammered copper, less than one-eighth of an inch thick, and it has a ridge on each edge to stiffen it, and act as a defence against rain driving in.
I have been the more particular in describing this, not only on account of the great comfort I have experienced under it, but also because the Hartwell Dome may very fairly be termed the first development of that latent talent for observatory engineering in Mr. May, which has since been so unequivocally manifested in various undertakings for the Astronomer Royal, Mr. Barclay,
himself, and others. When I planned my own revolving-roof at Bedford, economy led me to construct it of a straight-lined conic shape, instead of fashioning the ribs to a semi-circular bearing, as then generally obtained. But in this for Hartwell, where it was more imperatively necessary to consult appearances than with me, it was determined in conclave that it should be truly hemispherical, should move upon three balls, and should open by a single copper-shutter from the zenith to the wall-plate. And the present efficient and durable structure was the successful consequence, in which the roomy space, the strength, and the revolutionary principles are all equally admirable. This is proved by the fact that, though the dome has been completed about fifteen years, it has never required repair or alteration, nor has the wet penetrated anywhere. None of the swags between the points of support formed by the three balls, which a weaker construction would have had to endure, have been known: and, from the firmness and accuracy of the wallplate, these balls were found so little accelerated or retarded respectively that the roof was only lifted, for the first time since it was placed, on the morning of the 21st of September, 1850, there even then requiring but little adjustment. The mode of raising the roof a few tenths of an inch, just enough to release the balls whenever it is required to restore them to an equi-distant position from each other, is by a simple yet efficient contrivance of Mr. May's. It is by means of two stout little iron windlasses, unattached, and only used when necessary: they each consist of a pair of arched plates resembling the head of a large hammer, with a stout screw having a moveable capstan-head turned by a lever, thus affording great power of lifting in a compact form.
But while the tower was in full advance an unexpected interruption occurred, which temporarily gave the fine equatorial room to the five-foot achromatic telescope and its tripod stand. This was owing to Mr. Dollond's announcing that, on rigidly testing the large object-glass, he would not recommend it, as quite worthy of so expensive a mounting as had been proposed; and on our going to his house, the painful fact was proved. Still, though it did not attain the acme of excellence, it was so far good as to warrant
a proper apparatus; I therefore proposed a new and less responsible mode of calling it into action, which was about being adopted, when another circumstance changed its destiny. About the middle of the year 1836, I began to perceive that the object for which I had erected an observatory at Bedford had advanced to within a limited time of its proper accomplishment; and Dr. Lee, who had frequently made use of my great telescope, and was well acquainted with its high qualities, became a candidate for its possession when the materials for my intended astronomical work should be completed. This arrangement was the sooner made, because a future continuance of its use in prosecution of certain sidereal inquiries was most kindly and considerately conceded to me. When therefore my round of observations was complete, and affairs called me into Glamorganshire, my telescope was promoted from its humble location at Bedford to the splendid tower of Hartwell. This very fine instrument has been duly described, both in the Memoirs of the Royal Astronomical Society and in my Cycle of Celestial Objects; but the manner of my becoming possessed of the object-glass is best told in the words of Sir James South, who, in a letter to me from Camden Hill, of the 5th of October 1829, says
"I have brought with me from the continent a piece of flint-glass, upwards of twelve inches diameter, which I am going to have made into an object-glass. I recollect having promised you the refusal of my six-inch one, in case I resolved to part with it; such is now my intention. If therefore you are desirous of having it, pray let me know, as till I hear from you I shall not say anything about it to any one else. I do not mean to part with it for less than 2201. You will also remember that it has no stand, its clear aperture is 5.9 inches, its tube is of brass unpolished, it has a finder of large dimensions, is provided with illuminator and light-regulating apparatus, and has no eye-pieces. I believe it to be Tulley's chef-d'oeuvre; of this however it becomes me to speak with caution, as the instrument is new. Admiral Rossel desires his regards to you and Mrs. Smyth, as does also Baron Zach.”
A good six-inch object-glass was in 1829 almost unique; and as I had had an opportunity of testing the performance of the one in question at Mr. Tulley's, at Islington, I esteemed myself fortunate in securing the prize. The
mounting and equipment were instantly proceeded with; and, though science and practice have successfully laboured to improve equatorial movements since that time, I have every reason to be still satisfied with its simplicity, stability, and general performance. The principle is that of Mr. Jonathan Sisson's equatorial-sector, as described by Dr. Vince; in which an artificial polar-axis is placed parallel to that of the earth, with the hour-circle adjusted perpendicular to it, and a true collimation ensured. By thus obtaining a revolving axis in the same direction with the terrestrial one, the attached telescope readily follows any celestial body in its arc of revolution, without the trouble of repeated adjustments for the continual alterations of elevation, which attends all altitude and azimuth methods of mounting.*
Mr. Bishop's large telescope in Regent's Park, and Lord Wrottesley's in Staffordshire, are not only mounted on the same model, but are actually furnished with hour and declination circles cast from the moulds which were cut for me; and they differ so little in other dimensions that the three may almost be termed counterparts of each other, in a conventional point of view. As there is nothing like figures for nailing an assertion, the several sizes may be here shewn:
Now the first of these instruments, in the hands of such adepts as Dawes and Hind, has fully proved the excellence and facility of its working, in measuring many of the closest double-stars in the heavens, in picking up new
* In usual parlance, this species of instrument has Sisson's name attached to it, because Dr. Maskelyne considered the one made for him by that workman, to be an improvement upon that made by Henry Hindley at York, in 1741. But a polar-axis, though without the appendage of graduated circles, was used by Christopher Scheiner so far back as the year 1620.