SCIENTIFIC SUMMARY. 
191 
matter, such as a method of verifying- Jamin’s compensator with great 
delicacy. 
Distinguishing Lights for Lighthouses forms the subject of a communica- 
tion from S$r W. Thomson to the Times. He recommends : (1) A great 
quickening of all revolving lights. (2) The application of a group of dot- 
dash signals to every fixed light. (3) The abolition of colour as a distinction 
for lighthouse lights, except for showing dangers, channels and ports by red, 
white, and green sectors. Of about 120 revolving lights on the English, 
Scottish, and Irish coasts, there are in all eighteen in which the periods are 
ten seconds or less, and the times of extinction seven seconds or less. In 
these quick revolving lights, the place of the light is not practically lost, 
during darkness ; the eye, sweeping deliberately along the horizon, with or 
without the aid of a binocular, 1 to pick up the light/ passes over less than 
its own field of view within the period of the light, and thus finds it almost 
as surely as if it were fixed. What a contrast to the ordinary minute-period 
revolving light ! 
The distinction by colour alone ought to be prohibited for all lighthouse 
lights, on account of its liability to be confused with ships’ and steamers’ 
side-lights. Southsea Castle, with its red and green port and starboard side- 
lights, seems as if actually planned to lure an unsuspecting enemy to destruc- 
tion. His proposal is to distinguish every fixed light by a rapid group of 
two or three dot-dash eclipses ; the dot of about half a second duration, the 
dash three times as long, with intervals of light, about half a second each, 
between the eclipses of the group, and of five or six seconds between groups. 
Siemens's Differential Electric Lamp has one carbon attached to the end 
of a lever joined to a pair of iron cores, which are free to move up and down 
in two solenoids. One of these has large wire of small resistance, forming 
part of the lamp circuit. The other is a coil of smaller wire, offering 
greater resistance. It is in a circuit external to the lamp, joining the con- 
ductors and excluding the carbons. When the former is excited, it draws 
in its core, and the points of the carbons are separated; when the latter, 
they approach one other. The distance will thus be adjusted automatically, 
so as to maintain constant action. 
Connection of Surveys has been recently accomplished by M. Perrier, 
between Algiers and Europe. He found that from all the trigonometric 
points of first order about Oran, the loftier peaks of Sierra Nevada were 
visible in clear weather. The stations chosen in Algeria were the summits 
of Mount Filhaoursen and Mount M'Sabiha, west of Oran ; in Spain, those 
of Mounts Tetica and Mulhacen — the latter the most elevated point in the 
kingdom. The signals were to have been given by means of solar reflectors 
and powerful lenses, over a distance of 270 kilometres, but they failed 
utterly. Preparations had, however, been made for the employment of the 
electric light, and on the summit of each mountain a Gramme’s machine had 
been established. On August 20 the lights were displayed all night. It 
was not until after twenty days that one after another they became visible 
even to the naked eye. That on Tetica, nearly 270 kilometres distant, about 
equalled a Ursse Majoris, which rose near it. We have now trigonometric 
measurements of an accurate nature from lat. 61° in the Shetland Islands, 
to lat. 34° on the southern frontier of Algeria. 
