416 POPULAR LECTURES AND ADDRESSES. 



separated by equal intervals of light in the group, 

 and the groups of eclipses following one another 

 after intervals of not less than eight seconds of 

 undisturbed bright light ; next groups of two or 

 three short and long eclipses, the short eclipse one 

 second, the long eclipse three seconds, the interval 

 of light between the eclipses of a group one second, 

 and the interval of undisturbed light between the 

 groups of eclipses not less than eight seconds. I 

 fixed upon the time one second, because, after 

 many trials of mechanisms to produce the eclipses 

 I found that I could produce all the groups of 

 eclipses at the rate corresponding to one second for 

 the short eclipse by a simple and inexpensive 

 machine applicable to any lighthouse, large or 

 small, and of any variety of optical arrangement, 

 whether merely with condensation to the horizon, 

 or with the additional appliances required to con- 

 dense into a particular azimuth. 



A machine fulfilling these conditions is now at 

 work in the college tower of the University of 

 Glasgow, performing the short-long-short of the 

 following table for four hours every evening. It 



