Nov. 19, 1885] 



NA TURE 



57 



high. At Bedford the transit room contained a transit 

 circle and a transit instrument, with a clock so placed 

 that it could be used with either, as wanted ; but an 

 ordinary amateur will only need to have one meridian 

 instrument, and the surplus space may advantageously 

 be partitioned oft' to form a calculating-room, or the 



space may be used as an ante-room, and the entrance 

 door put there, and not on the north side, as at Bedford. 



It will now be convenient to describe the several parts 

 of an observatory more in detail. 



TIic Equatorial Room. — The equatorial being the 

 principal instrument in every amateur's observatory, the 

 provision made for its accommodation deserves attention 

 first. It is not an uncommon practice to arrange that 

 the floor of the equatorial room shall be 2 feet or 3 feet 

 below the level of the adjoining room, and where a large 

 equatorial is worked with a small transit instrument used 

 merely for setting the clock, and economy and difficulties 

 of site have to be considered, a sunken equatorial room 

 may be unavoidable. But all the same the practice is 

 highly inconvenient and ojectionable. An observer should 

 be able to move rapidly from one part of his observatory 

 to another in the dark, and without having to think of 

 steps up or steps down. Moreover, in order to secure free 

 internal ventilation nothing more substantial than a green 

 baize curtain should separate the equatorial room from 

 the transit room, and it is obviously not safe to use such 



Fig. 3. — Diagram showing ribs of a dome intended to be covered with 

 copper or sheet iron. 



a curtain where it will conceal a difterence of level of 2 

 feet or more. 



Fig. I contemplates an equatorial of what is called the 

 " English " form, with two separate piers for the support 

 of the polar axis ; but this construction of equatorial has 



become almost obsolete, owing to its numerous practical 

 disadvantages, and the " German " form, with one pier 

 and pillar, centrally placed, is now all but universally 

 used, at least by amateurs. 



The construction of a roof for an equatorial room 

 (technically called the " dome," whatever may be its 

 precise form) is a great crux to the intending builder of 

 an observatory. Theoretically the hemisphere is the 

 proper form, and roofs truly hemispherical are occasion- 

 ally met with ; but they are e.xtremely troublesome and 

 expensive to m.ike, and can only be tackled by pro- 

 fessional engineers. 



Fig. 3 represents the skeleton framework of such a 

 dome of large dimensions, before the sheet copper, or 

 other material to be employed in covering it, has been 

 been put on. Of late years, especially for large observa- 

 tories, " drum " domes have come much into use as com- 

 paratively easy to construct, and capable of being made 

 strong and watertight ; but they offer much resistance to 

 the wind, and architecturally are bound to be ugly. 



For the purpose of protecting the smaller sizes of equa- 

 torial, say those from 4 inches to 7 inches in aperture, a 

 polygonal dome is recommended. Or, in the case of 

 equatorials of the smallest size, say from 2i to 5 inches, 

 the roof of the equatorial room may be flat, and arranged 

 to open by sliding it to one side. Such a sliding roof 

 should not be quite mathematically flat but should have 

 a slight inclination, to throw off the rain. 



Whatever be the form of the dome chosen the problems, 



Fig. 4.— Wooden Observatory erected at Eastbourne in 1834- 



how to uncover a slit in it, and how to move the whole of 

 it, are matters which require in all cases careful con- 

 sideration. Where the dome is a large one, say more 

 than 12 feet in diameter, the shutters which close the slit 

 should slide. They may slide laterally on a suitable 

 staging (as in Fig. 4), or they may slide up and down. 

 The latter is a very convenient expedient, especially when 

 the observatory is to be erected in a situation exposed to 

 strong winds, or when the telescope is to be much used 

 on the sun ; for the observer can open just so much space 

 as will uncover the whole aperture of the telescope, and 

 can keep himself and the greater part of his telescope 

 protected from the direct impact of the wind, or the direct 

 rays of the sun, as the case may be. When arranged in 

 the best form the shutters will be three or four in number, 

 each protecting a third or a fourth of the slit, measured 

 vertically. Each shutter must have its own rabbet, and 

 its own ropes and pulleys, in order to enable the observer 

 to open at one time only so much of the whole slit as is 

 necessary to enable him to scrutinise the particular por- 

 tion of the heavens which he desires to examine. The 

 advantage of thus being able to shelter himself and his 

 telescope will soon be appreciated in windy weather, or 

 under a meridian sun by the owner of an observatory 

 fitted with sliding shutters. 



Another important matter is the question of the bear- 

 ings on which a dome is to be mounted. Large domes 

 can only be made to move with facility by the aid of 

 mechanical appliances which are often in practice both 

 complex and cumbersome, and needing much muscular 

 effort on the part of the person who has to move the 



