HARDWICKE'S SCIENCE-GOSSIP. 



255 



raise or lower it to any required height, move it 

 in any direction, or alter the inclination of the back. 

 In some observatories the floor itself can be raised or 

 lowered at will. 



Now let us examine the instrument itself. Its 

 mounting, as we said, in no wise resembles our 

 simple stand : we are told that it is an Equatorial. 

 Who that has a telescope and an ordinary stand 

 cannot recall some such experience as the following ? 

 He is eager to show an unscientific friend some 

 celestial object — it may be Saturn, that wonderful 

 planet with his belts, rings and moons, a really good 

 view of which is so seldom obtained in England. 

 He gets it into the field of view — as •that part of the 

 sky visible in the telescope is called — and tells his 

 friend to look. His friend, after various ineffectual 

 efforts, gets his eye into the proper position, glances 

 up the tube and immediately exclaims, " It is gone !" 

 It has not gone, but all the while that he has been 

 performing his evolutions the earth has been steadily 

 rotating on its axis as usual, carrying with it the 

 telescope and the would-be observer. The telescope 

 is slightly moved, another attempt made and this 

 time, perhaps, with success. Saturn, with all his 

 retinue, passes majestically across the field of view. 

 Every few seconds the position of the telescope has 

 to be altered in a direction contrary to that in which 

 the earth is moving, and, if the stand is not well 

 made, every movement will be attended with 

 vibration. Such a state of things will never do when 

 long-continued, careful observations are to be made. 

 It is to do away with such inconveniences that the 

 equatorial or parallactic form of mounting was 

 devised. Of this there are two sorts, known re- 

 spectively as the English and the German pattern. 

 The latter is the more generally met with, since 

 with it the telescope can be easily pointed to any 

 part of the sky, which is hardly the case with the 

 English form. If we watch the motion of a heavenly 

 body for a few hours, we shall find that the path it 

 appears to follow is part of a circle. Let us note, 

 for instance, the middle one of the three stars 

 forming the belt of Orion, sometimes called the 

 Tailor's Yard, and watch it from its rising. We 

 shall see that the path it describes, as it gets higher 

 in the heavens, is not vertical. Now let us try 

 to follow it with a telescope mounted on a simple 

 stand. We shall very soon find that we are unable 

 to keep the star in the field of view with either a 

 horizontal or a vertical movement, but we must 

 combine the two. If, however, we tilt the pillar of 

 the stand in a northerly direction until it points to 

 the north pole — very near which is the pole-star — we 

 shall be able to follow the star in question by means 

 of one movement only, but that neither horizontal 

 nor vertical. Let us try the experiment with any 

 other star, and we shall meet with equal success ; 

 the middle star in Orion's belt was selected simply 

 because it is close to the equator, and for that reason 



somewhat better for our purpose. We have, in fact, 

 constructed a simple equatorial stand. 



But still something more is needed. We want our 

 hands to be free, and not engaged in constantly 

 imparting movement to the telescope. Skilful 

 mechanicians again come to our aid, and, by means 

 of clockwork attached to the telescope, the whole 

 instrument is kept moving at exactly the rate 

 required to counteract the motion of the earth. 

 With this addition to our equatorial, let us turn the 

 telescope to any heavenly body that we are desirous 

 of scrutinising, and it will remain, as if it were fixed, 

 in the centre of the field of view, so long as the 

 clockwork is kept going. Clockwork of this descrip- 

 tion has now been brought to a very high degree of 

 perfection. A similar arrangement is sometimes 

 adopted for the purpose of moving the floor round. 



Attached to the main tube of the telescope we 

 notice one or more smaller ones, called Finders. 

 On looking through one of these we shall find a cross 

 of fine wires dividing the field of view accurately into 

 four quadrants. When any object is seen at the 

 centre of this cross, it will be found also in the centre 

 of the field of view of the large telescope, for the two 

 instruments are placed exactly parallel to each other. 

 With a large telescope it is very difficult to point 

 accurately to an object ; even when the aperture is 

 only a very few inches, a finder is an absolute neces- 

 sity. There are, also, many other articles attached 

 to the great tube, the various uses of which are at 

 present a mystery to us. 



Having pursued our examination thus far, we are 

 naturally anxious to see some of the wonderful sights 

 in the heavens that we have heard of. We, there- 

 fore, crave and gain permission to spend some hours, 

 both by night and by day, at different times of the 

 year, within the observatory with our guide. We 

 shall most likely be told not to choose nights, how- 

 ever clear, when the wind is easterly. Clear defini- 

 tion is never obtained when the air is very dry. In 

 our climate the best nights for astronomical observa- 

 tions are in September, and the early hours of the 

 morning are always superior to the late hours of the 

 evening. Finding ourselves beneath the dome, a patch 

 of starlit sky is exposed to our view by the opening 

 of the shutter. When we had our small telescope 

 in the garden, we could easily recognise the familiar 

 constellations and turn the instrument to the stars 

 we wished to examine, but now we have only a 

 limited amount of sky-room within view, and an 

 element of difficulty is thereby introduced. The 

 finding of a particular object, unless it be the sun or 

 moon, is not a matter to be easily accomplished by 

 the mere shifting of the telescope. If we wish to 

 describe the position of any place on the surface of the 

 earth, we state its latitude and longitude, that is, we 

 say that the place is situated on a certain imaginary 

 line, so many degrees north or south of, and parallel 

 to, the equator, at a point where it is crossed at right 



