CH. xxxil. ADVANCES IN ASTRONOMY. . 3" 



Again, self-registering instruments are now at work in 

 every part of the world, recording events at every instant 

 with unvarying accuracy. Clockwork regulates the move- 

 ments of the telescope itself, while microscopes enable the 

 observer to read off the most delicate measurements. And, 

 what is of still greater importance, photography enables 

 us to obtain permanent records of passing astronomical 

 phenomena. Thus photographs were taken of the ingress 

 and egress of Venus during the late transits, and though 

 these were imperfect, yet there is little doubt that such 

 attempts will be successful in the end, and hourly photo- 

 graphs are now being taken of the sun whenever it is 

 visible. 



With all these appliances many features of the heavenly 

 bodies are observable which were before indistinct. On the 

 nights of August n and 17, 1877, Professor Asaph Hall dis- 

 covered with the Washington telescope two of the smallest 

 heavenly bodies (meteorites excepted) which have ever 

 yet been seen. These are two satellites revolving round 

 the planet Mars. It is difficult to be certain how large 

 they are : the one which is farther from the planet is 

 the smaller, measuring probably, according to Professor 

 Pickering, about six miles across, and it moves round 

 Mars in 30 hours 17 minutes 53 seconds. The inner 

 one is brighter, and measures probably about seven miles 

 across; its period of revolution is 7 hours 38 minutes. 

 These two satellites are named Deimos and Phobos, or Fright 

 and Flight, because Homer makes these attendants upon the 

 God of Battle. They are so near to Mars that if there be 

 living beings on that planet with eyes and telescopes like 

 ours, they will be able to tell whether their moons are in- 

 habited, for the inner one is only 4000 miles distant, 01 

 sixty times nearer Mars than our Moon is to us. 



