434 SCIENCE PROGRESS 



seriously to study the subject. In other words, it is an instru- 

 ment for popularising the study of astronomy. 



Such an instrument appears to have been made for the 

 first time early in the eighteenth century to illustrate the motion 

 of the moon, other features being soon added. One was made 

 for the Earl of Orrery, who played the part of patron and 

 encouraged its use. The new instrument was therefore called 

 by his name at the instance of Sir Richard Steele. Interest in 

 science being general, it soon became fashionable, and evidently 

 one must have been available for Wright's use in Derby. Many 

 orreries were operated mechanically, but there is no trace of 

 mechanism in the picture, and the movements were doubtless 

 made by hand. The horizontal circle represents the plane of 

 the ecliptic in which the earth moves, and on it are depicted 

 the signs of the zodiac and the months of the year. The 

 orbits of the planets were probably traced on the horizontal 

 surface below the ecliptic. The inclined circle is the Equator, 

 and the great circles at right angles to it meet in the Pole. These 

 enable the position of a planet or star to be determined. Simi- 

 lar instruments are still made, although their popularity is 

 a thing of the past. The figures in this picture are portraits 

 of Wright's friends. 



The position of the lamp is worthy of notice. The lamp 

 itself is hidden by a figure in the foreground, whilst it 

 strongly illuminates the greater part of the apparatus and 

 the faces of the company. Its reflection is seen in the fore- 

 ground. 



The second picture bears the title " An Experiment on a 

 Bird in the Air-Pump." The air-pump had been invented 

 by Otto von Guericke, a magistrate of Magdeburg, rather more 

 than a century previously. Von Guericke 's air-pump consisted 

 essentially of a copper globe to which a pump was attached. 

 The means of creating a vacuum had long been desired, and 

 the invention immediately attracted the attention of the leaders 

 of science throughout Europe. As a result mainly of the 

 labours of the EngUsh workers Boyle, Hook, Hawksbee, and 

 Smeaton, the latter a contemporary of Wright, it assumed 

 the form of the double-barrelled pump with the flat plate and 

 glass receiver as shown in the picture. In this form it was 

 an efficient instrument for producing what we should now regard 

 as low vacua. Von Guericke was also the inventor of the 

 Magdeburg hemispheres which appear in the foreground of the 

 picture. It is said that when placed together and exhausted, 

 two teams each of fifteen horses failed to pull them asunder. 

 In the picture the receiver is shown mounted on a tall pillar, 

 a quite usual feature in air-pumps of the period. 



Whilst the various branches of mechanics were well developed 



