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POPULAR SCIENCE REVIEW. 
this point of suspension through a vertical galvanometer. The shock- 
receiving part is placed underground to avoid the interference of winds or 
that of violent detonations, the metal block being set upon a wooden pile 
driven some distance in solid earth. When properly set, a single make and 
break contact of this kind is so sensitive that the impact of 3 pounds of 
stone, falling from a height of 5 ft. upon the ground, at a distance of 50 ft. 
from it, moved the needle of the galvanometer very determinately. The 
intervening ground was clay. 
Coal-gas for Lighthouses. — Mr. J. Wigham recently gave a lecture in 
Dublin on this subject. He said that coal-gas was first used in lighthouses 
in 1865, by Mr. Samuel Bewley of the Irish Board of Lights, who tried 
some experiments at Howth. The first burner used was called the “ crocus.” 
The principles involved in this burner, and the means taken to economise 
the gas, proved that by the crocus there was an immense saving, taking light 
for light, as compared with the gas usually used in our houses. The lecturer 
then alluded to the economy as compared with oil. There was a saving of 
about 50/. per annum on each lighthouse in which the gas had been tried, 
and, in the case of intermitting lights, the difference was much greater as 
regards economy. Dr. Tyndall has been sent down to investigate the whole 
matter at Howth, and that gentleman had reported favourably. There were 
five lighthouses at present on the Irish Coast illuminated by gas, and they 
are about to try it on two of the English lighthouses. In speaking of the 
electric light, Mr. Wigham said that though the latter was very intense, yet 
it was deficient in quantity, and it was not so good as coal-gas for penetrat- 
ing fogs. Mr. Wigham then proceeded to explain the mechanical arrange- 
ments and the construction of the lenses. — Royal Dublin Society , March 18, 
1872. 
Estimating the Intensity of Light. — The Chemical Neivs , April 12, quot- 
ing from an American journal, states that Dr. Vogel proposes nitroprussid 
iron as a suitable salt for determining quantitatively the intensity of light. 
For the preparation of this reagent, dissolve chemically pure oxide of iron, 
best obtained from the oxalate, in hydrochloric acid, and evaporate nearly 
to dryness to expel the excess of acid ; and after filtering, add an aqueous 
solution of nitroprussidnatrium in proportion of three of the iron to two of 
the latter. There is usually a slight precipitate produced by this mixture, 
which can be collected on a filter ; but this operation must be performed in 
a dark room. We have now a liquid excessively sensitive to the action of 
sunlight. By exposing a small quantity of a known specific gravity to the 
action of light, a precipitate of prussian-blue will instantly begin to fall ; 
and, on redetermining the sp. gr. in the dark chamber, its decrease will be 
found to be proportional to the precipitate ; and we have thus the data for 
measuring the intensity of light. It was found by Dr. Vogel that the 
liquid, exposed for forty-eight hours before a kerosene lamp, was not in the 
least affected, but a piece of magnesium wire, when burned, immediately 
produced a precipitate. By employing a long instrument graduated in milli- 
metres, it would appear to be possible to measure the intensity of the light 
by the number of millimetres occupied by the precipitate. The invention 
has an important bearing upon photography. 
Sow to bend Glass Tubes so as to fit Apparatus . — Mr. J. Laurence Smith 
