58 



NA TURE 



[_Nov. 19, 1885 



dome. Where the weight of this does not exceed a ton, 

 a set of grooved wheels running ina concave wall-plate of 

 iron generally works well. For weights be)-ond this, 

 special mechanical appliances must be used, which it is 

 foreign to my present purpose to treat of. On the other 

 hand, light domes — by which is meant domes up to, say, 

 half a ton — are best dealt with by being mounted on iron 

 balls (cannon balls in fact) travelling on a circular wall- 

 plate, and kept in place by an upper plate, the arrange- 

 ment being such as is indicated in Fig. 5. 



The ironwork may be simplified in character and 

 lessened in weight if the upper plate, which in Fig. 5 is, 

 like the lower one, a solid casting, is replaced by two 

 detached rails about an inch square in section and placed 

 about 3 inches apart. The balls need only be three in 

 number where the diameter of the dome does not exceed 

 10 feet. If the diameter is greater than that a fourth ball 

 may be desirable in order to distribute better the weight, 

 and lessen the risk of the framework of the dome being 

 strained. The diameter of tlie balls may be 4 or 5 inches 

 (say 24-pounder or 32-pounder balls), and the more truly 

 sphericalthey are the less the friction, and consequently the 

 less the muscular effort, required to impart motion to the 

 dome ; and to this it may be added the less likely are the 

 balls to approach one another after being some time in 

 use and so in a sense dismount the dome. When this 

 does happen the dome must be slightly prised up by means 



Fig, 5. — Section of beaiiii^s for a dome (Bedford Observatory) 



of a lever or jack, and the balls separated and set at a 

 distance from one another of 120'' or 90°, according as 

 there are three or four of them. 



Where the dome is a light one, mounted on cannon 

 balls, motion may be imparted to it by the simple process of 

 pushing a long and strong handle which descends from 

 the roof to a sufficient distance towards the iloor ; in 

 other words, which is 4 feet or 5 feet long. Where a 

 handle of this sort is used it should be affixed to the 

 dome by strong screws or bolts, exactly opposite the 

 shutters which cover the main opening, because when so 

 placed the observer can grasp the handle and bring the 

 openings exactly to that part of the heavens to which he 

 has pointed his telescope, and can be sure that he has 

 done so. In this facility of being able to watch how far 

 the dome is moved resides the great advantage of the 

 fixed handle ; its disadvantage is that the observer in 

 moving the dome has to follow it himself by walking 

 around on the floor. To obviate this inconvenience, such 

 as it is, some prefer a fixed wheel permanently attached to 

 some one place in the wall of the observatory, and having 

 cams in its periphery to catch suitable pins attached at 

 short intervals to the inside circumference of the revolving 

 dome. 



Whatever may be the form of the dome, it is evident 

 that in plan it must at the bottom be circular, and that 



the wall-plate must be circular also, and of the same 

 dimensions. But the plan of the equatorial room, as 

 regards its walls and floor, is another question. Where 

 the room is large, say 15 feet or 20 feet, or more, in 

 diameter, it will be best that it also should be circular, or 

 perhaps octagonal. Where, however, the dome is not 

 more than 12 feet across, and consequently the whole 

 establishment is on a small scale, there are great advant- 

 ages in making the equatorial room square. In such a 

 case the corners will be found very useful for various 

 purposes : for instance, in one a desk or writing-slab may 

 be fixed ; in another, the clock ; in a third, a lamp ; 

 whilst the fourth corner may take a chair or a stool. In 

 other words, the corners become available as places of 

 refuge for things and persons whilst the observer is turn- 

 ing the dome round from one part of the heavens to 

 another. Moreover, the cost of building a scjuare room 

 is less than the cost of building a polygonal one, because 

 the difficulty is less, be the material brick or wood. If 

 wood is employed for the walls of an observatory, it will 

 in all cases be desirable to place the frame on a dwarf 

 wall of brickwork rising at least 2 feet above the general 

 level of the ground. 



The floor must be supported on joists^ trimmed so as 

 to form square frames around the piers which are to 

 carry instruments. This will enable the floor-boards to 

 be fixed firmly, yet quite clear of the piers, and will pre- 

 vent tremors, caused by persons passing over the floor, 

 being conveyed to the piers, and so to the instruments. 

 -A free circulation of air must be secured by means of 

 small brass ventilating gratings suitably disposed around 

 the floor near the walls. 



Alaking due allowance for the different purposes for 

 which it is to be used, many of ther remarks just made 

 with respect to the equatorial room will apply also to the 

 transit room. The main part of the roof is a fixture, but 

 an opening about i foot 6 inches wide has to be made 

 right across the top, and to be continued into the north 

 and south walls from the eaves downwards towards the 

 floor, so as to enable the observer to sweep the meridian 

 with the transit instrument from the south horizon 

 through the zenith to the north horizon. The openings 

 must be protected by shutters, which may either slide or 

 lift. For large observatories Grubb has devised a form 

 of balance shutter which swings, and is said to work well. 



In cases where the top transit shutter, which consti- 

 tutes part of the roof, is in the form of a flap and lifts, it 

 must be counterpoised by a weight or weights travelling 

 up and down inside the room. '1 he vertical shutters must 

 be treated as casements, and be fitted with handles and 

 fastenings accordingly. The remarks made in speaking 

 of the equatorial room as to the advantages offered by 

 sliding shutters or sashes, apply equally to the case of 

 sliding shutters for a transit room. 



Light should be obtained for an observatory by inde- 

 pendent windows, and not, as in Fig. 2, by panes of glass 

 inserted in the shutters ; for glasses are very apt to get 

 broken by the constant moving of the shutters. 



The transit instrument as such I need not describe in 

 detail, but it may be worth while to show how a transit 

 instrument is mounted where space is no object, and the 

 instrument is intended for the determination of Right 

 Ascensions rather than for the commonplace purpose of 

 setting a clock. 



The transit instrument at Bedford consisted of a tele- 

 scope of 3i feet focal length furnished with an object-glass 

 whose aperture was 3j inches ; the telescope was sup- 

 ported by broad cones forming an axis 28 inches long, 

 the pivots of which rested on covered Y"s offering a surface 

 of polished Brazilian pebble an inch in bearing, and 

 which (owing to their bases being hemispherical and 

 working in corresponding sockets) held their propor- 

 tionate weight, as well as ensured the axis of the pivots 

 being always strictly in the same right line. The Y's 



