6o 



NATURE 



\Nov. 19, 1885 



being tampered with and the mark displaced from the 

 meridian. On the sliding plate there was soldered a 

 square piece of silver exhibiting a well-defined black cross, 

 the centre of which was to mark the actual meridian. As 

 this cross taken by itself hardly afforded sufficient 

 vertical length for comparing the wires of the transit a 

 small circle of silver (with a black dot in its centre) was 

 placed above the cross as an auxiliary mark. This silver 

 circle, like the larger silver plate below, was capable of 

 lateral motion by means of capstan-headed screws which 



d by Ramsden. 



could be removed when the dot had been brought exactlv 

 over the cross below. The stone to which the mark was 

 fixed was firmly morticed into a dwarf pier, to guard 

 against lateral movement, and the whole superstructure 

 was firmly bedded on a solid substructure sunk into the 

 earth. It is of the utmost importance to guard against 

 settlements likely to cause any lateral movement, for it 

 must be remembered that with a 50-foot radius a dis- 

 placement of about 3-ioooths of an inch is equivalent to one 

 second. The remaining and important part of the 



arrangement at Bedford was a 4-inch lens of 49^ feet 

 focus, being exactly its distance from the diaphragm. 

 This lens was mounted in a brass collar, and having been 

 attached by screws to a plate of cast-iron, was let into 

 the wall of the transit window in a line with the transit 

 instrument and the meridian mark. It is evident that the 

 rays of light from the meridian mark become parallel after 

 passing through the lens, and that the diaphragm can 

 therefore be viewed through the telescope of the transit 

 instrument as adjusted to solar focus. Another con- 

 sequence of the rays being rendered thus parallel, is that 

 no parallel motion of the transit axis would cause a 

 change in the place of the object seen, so that the 

 meridian is a line drawn from the diaphragm through the 

 axis of the lens : and provided that these two points 

 remain rigidly permanent, they oft'er all the advantages 

 of a very distant meridian mark. And after all, a distant 

 mark when obtainable can still be used as a check to the 

 home mark. It will often happen that an observer will 

 be able to find at the distance of a mile or two, or even 

 of several miles, some well-defined line or point — e.g. a 

 window sash, or the pinnacle of a church, or a piece of 

 squared stone, which will serve him as a meridian mark 

 for the simple reason that it lies in the meridian of his 

 transit instrument. 



Clocks. — A clock is a very important article of furniture 

 in every observatory. 



!'■':':!! 



Whilst a proper sidereal clock showing twenty-four 

 hours is what an amateur should have, he can very easily 

 make shift with a much less pretentious time-piece, 

 especially if his equatorial is provided with the best 

 modern form of driving clock which only requires to be 

 set once, or occasionally, during an evening's work. 

 Indeed all that is essential in such a case is really a good 

 dming-room clock (with its pendulum adjusted to sidereal 

 time) which once set at the commencement of an evening 

 by means of a transit instrument can be depended upon 

 to mamtain a tolerably even rate for half a dozen hours. 

 The price of sidereal clocks for observatory purposes has 

 been much reduced of late years, and from 20/. to 30/. 

 will now command a fairly good one. 



Where an observatory includes a transit room the clock 

 should of course be placed so as to be visible both to an 

 obser\'er sitting at the transit instrument and facing the 

 direction in which transits are most usually taken (that is, 

 for the northern hemisphere, south) and also visible to an 

 observer working with the equatorial. This desirable 

 combination makes it expedient that the equatorial room 

 should be at the west end of the buildings ; but local 

 reasons connected with the site of the observatory may 

 not always render this possible. 



For the clock there should be provided a stone pier 



constructed and isolated with much the same precautions 

 as those already suggested in respect of the piers prepared 

 to carry the telescope. 



On the top of the clock case there is sometimes placed 

 a " Hardy's noddy.'^ This is a small and sensitive 

 inverted pendulum inclosed under a glass bell and standing 

 on a frame provided with three adjusting screws to level 

 it. The use of the noddy is to discover whether the 

 pendulum of the clock imparts any motion to its supports. 

 But this is a refinement with which in a general way 

 amateur observers need not concern themselves. 



Meteorological Instruments. — .Although an astronomical 

 observatory is one thing and a meteorological observatory 

 is altogether another thing, yet every astronomical 

 establishment should be provided with a few of the more 

 ordinary meteorological instruments, even though their 

 owner does not profess to be a meteorologist. All astro- 

 nomical observations are in a measure affected by 

 changes in the temperature and humidity of the air ; 

 consequently, a self-registering maximum and minimum 

 thermometer, a hygrometer, and a rain-gauge should be 

 regarded as indispensable accessories to every observa- 

 tory. No doubt, also, the desirability of having a 

 barometer will naturally suggest itself, though its astro- 

 nomical usefulness is very small indeed— by which I 



