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OBSEBVATORIES.

The degree of perfection already reached by optical instruments demands a more careful study of their surroundings, including the question of all influences that may affect their free and perfect performance. Of these influences—after the almost inscrutable differences of atmospheric disturbance in different places—the effects of our buildings upon the definition of our telescopes is the most important consideration.

With the large apertures now in the hands of so many amateurs, the disturbance of the visual rays is so great and annoying, and so large a part of it is manifestly due to the observatory itself, that some of the most enthusiastic observers prefer the open canopy of the skies to the comfort and shelter of a revolving roof, when it is purchased at such a price. But it is now confidently hoped that these observatories, based upon the experience of years, will go far to dispel an error, which tends not only to destroy that perfect personal comfort so necessary to the free exercise of the delicate senses of an observer, but the due protection which a good telescope deserves.

It may be boldly asserted that the construction of observatories has proceeded, hitherto, upon a wrong principle. The instruments are generally, so to speak, smothered in the nests built for them, and almost “ killed by kindness.” Where expense is no object, solid structures of brick or stone are reared, with more or less massive domes, at a great expenditure of money and materials; herein lies the mistake, for it is now capable of abundant proof that the optical efficiency of observatories is in the inverse ratio of their solidity, and, provided the structure be capable of resisting all the storms that may beat upon it, the thinner it is the better it will fulfil its functions.

The two great desiderata in an observatory are—i, freedom from damp; 2, non-interference with heavenly rays. Both of these are entirely dependent upon temperature. (1) Damp is the condensation of moisture from the surrounding air upon any surface colder than itself—witness a glass of very cold water brought into a warm room; but more than this, a chief source of damp is the condensation of moisture from a portion of ground inclosed by solid walls, and covered over; and as artificial heat is manifestly inadmissible in an observatory, such a building, especially if it consists only of a ground floor, is simply a condensing chamber, and when any considerable rise of temperature occurs, as in a thaw, the walls run down with moisture, and the instruments are coated with dew, all of which is plainly attributable to the fact that the walls, &c., do not rapidly accommodate their temperature to that of the air, and they keep the instruments within them colder than they would otherwise be, and that for a considerable time. Again (2) as to the non-interference with

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