currents of air of unequal density are set in motion, which are prejudicial to definition (even in the use of small instruments), and absolutely fatal to the performance of large ones, especially of reflectors, a form of telescope which has, during the last ten years, again come extensively into use.

The definition of refractors is not so liable to disturbance by neglect in this respect, probably because the object-glass is so near to the roof of the building that there is a much shorter column of air over it than there is over the speculum of a reflecting telescope.

This observatory is of two storeys; the lower one (which is not necessary, but is always convenient) has been made for the purpose of raising the telescope sufficiently above surrounding objects, and is utilised for a laboratory and photographic room; by having the telescope so far above the ground the observing room is entirely free from damp.

The pier, which supports the telescope, rises from a firm foundation, and is carried through this room without touching either of the floors, so as to avoid the communication of

tremors, caused by persons moving inside the building, or by wind on the outside of it. The pier is hollow in the centre, in the usual way, and is open down part of one side, so that a well-ventilated space is allowed for the fall of the weight of the driving-clock.

The upper or equatorial room, which is of the form known as the drum dome, is circular, of 13 feet 6 inches in diameter, and is 10 feet high, and, with the exception of the lowest part, 1 foot in height, which is a continuation of the wall of the room below, consists of a timber frame-work covered with corrugated zinc.*

The drum rotates on twelve wheels, which are of 5 inches in diameter, and are firmly bolted to the top of the wall, and it is provided with two large openings by which every part of the sky can be reached. One of these openings is in the roof, and is 9 feet by 3 feet 6 inches clear aperture, and the other, extending nearly the height of the dome, is 8 feet by 3 feet 6 inches.

It will be apparent from this description that care has been taken to carry out, as far as possible, the principle alluded to at the commencement of this paper, viz., the maintenance of equilibrium between the inside and outside temperatures.

By covering the frame of the building externally with sheet metal, as night approaches the heat which it has acquired during the day will be rapidly radiated; and by having such large openings the temperature of the whole building, and the contained air, can be brought in a few minutes to within 1° Fahr. of that of the external air. This

* Technically known as "Italian zinc roofing. The zinc is manufactured in sheets, 8 feet by 3 feet, each having two ridges or corrugations running lengthwise, which reduce the width of the sheet to 24 feet, but at the same time add so greatly to its strength, that it is used for roofing without other supports than light rafters, set 15 inches apart, so as to fit into the corrugations. Many acres of dock-sheds, on both sides of the Mersey, are covered in this way.

result could have perhaps been even better attained by a construction wholly of iron, but from the experience of a previous observatory of that material, in which much annoyance was found in doing those little alterations and additions which are constantly required, the present mode was chosen instead, as being nearly equal theoretically, and much more convenient in practice; besides, it must not be forgotten that, however perfect in this respect the building itself may be, a large telescope and its mounting may absorb a quantity of heat, that cannot be immediately radiated.

The openings of the dome are closed by two movable shutters. The roof shutter, which from being so large would be difficult to move in other ways, runs on six small flanged wheels on iron rails, and these rails are carried on supports which extend beyond the building just so far as to allow the shutter to completely clear the opening in the roof, which is four feet longer than the semi-diameter of the dome-a plan which I find to answer exceedingly well, for the shutter is remarkably easy to open and close, and at the same time weather-tight.

A door which opens outwards on hinges closes the side aperture.

If these openings were made in the ordinary way, continuous, it will be perceived that the strength of the structure must be interfered with, for it would be necessary to leave a space of 17 feet by 3 feet 6 inches unsupported by any ties, and a defect known as sagging would be introduced. It is easily seen that if this should take place to only a very slight extent it would derange the fitting of the top shutter; so to obviate this, the side shutter is made to open, not in a line with the horizontal one, but at a point removed from it by about one-twelfth of the circumference; and by this the strength of the building can be made scarcely less than if only one of these openings had been required.

There are two inconveniences connected with this method, which are, however, more apparent than real. The first is, that as one shutter opening ends on the same horizontal line as that at which the other begins, there is a part of the sky not reached by the telescope; this is met by having a subsidiary shutter underneath the horizontal one. This shutter is of the same width as the opening, and about 2 feet in depth; it opens on hinges, and hangs down close to the inside, but as it is found to be rather in the way sometimes, it is proposed to put it on hinges that will allow of its being opened back outside.

The second inconvenience is, that sometimes the dome has to be rotated through the angle which separates the openings, if an object while being observed for a length of time happens so to vary in altitude that the change has to be made from one opening to the other.

In practice, I may say that the latter of these difficulties seldom occurs, and the former entails so little extra trouble in working, or expense in construction, that it is not a matter of practical inconvenience, while and I now speak after a year's constant use-the framework continues quite unaltered in shape, and none of the shutters have at any time exhibited any signs of flexure.

I believe that this mode of construction is a novel one, and I think it will prove even more useful in much larger observatories than in one of this size.

The base of the dome is made of three rings of pine, each 9 inches wide and 1 inch thick, well screwed together, and to its lower side is attached a rack-wheel, which is 13 feet 3 inches in diameter and 5 inches in width, made in fourteen segments. The rail which runs on the twelve grooved wheels that are fixed to the wall below, is in the outside of this wheel and in the same casting. By making the rack-wheel and rail in the same casting the stiffness of the dome is increased,

f

and a perfectly circular rail is secured, which makes its rotation very easy; it is turned by a pinion gearing into the rack-work, to which is fixed a wheel of about 18 inches diameter.

The study of Observational Astronomy differs from almost all other scientific pursuits, in that it is entirely dependent upon the state of the weather; and as in this part of the world the weather is much oftener unfavourable than favourable, it is requisite that the short available time should be economised to the utmost. Now, there is no better way of accomplishing this than by having everything connected with the telescope and the observatory arranged so that the least possible time is consumed in starting work. It is also well that all moving parts shall act smoothly and without the use of a great degree of force for it is impossible after any considerable muscular exertion to turn round and make a fine micrometrical observation.

In all the details which I have described, and they are only those in which any novelty of construction occurs, these points have been carefully considered.

The telescope for which this observatory was designed is an equatorial reflector, made by Mr. John Browning; the diameter of the silvered glass speculum is 94 inches, and the focus 6 feet 6 inches. Its performance is excellent, both for space-penetrating power and for the manner in which it divides the closest double stars.

Mr. G. H. MORTON, F.G.S., read a communication on "The Borings for Water at Bootle." As these Borings are not yet completed, the printing of the communication is deferred for fuller information.

Ladies were present at this Meeting.