NATURAL PHILOSOPHY. 215 



of light. Any digit may be expressed by an equivalent number of occulta- 

 tions and restorations of the light; thus, one eclipse and one restoration 

 would stand for the number one. The value of the digit, whether belonging 

 to the units, tens, or hundreds' place, might be indicated by occupations 

 preceded by shorter or longer intervals of light, as three occultations at inter- 

 vals of a second would express three units, then a pause of several, say three 

 seconds, then five occultations, would express five in the tens' place, then a 

 pause of say three seconds and two occultations would express the hundreds, then 

 a longer pause of say ten seconds, would show that the number was complete. 

 Thus the number of a light house might be repeated more than once in a 

 minute, even where the figures are quite high, and each light house would 

 continue the repetition of its own number. Such lights can be seen at least 

 as far as others which are not temporarily obscured, and by arranging the 

 numbers of the light houses along a coast, upon such a system, that the 

 adjacent lights shall have very different numbers, the figures representing 

 units, tens, and hundreds of the number not recurring in the adjacent lights, 

 the distinctions can practically be made very complete. For the world-wide 

 purpose of its inventor, but three digits are required. The mariner who 

 approaches Sandy Hook, for example, would see constantly repeated number 

 one, a flash for a second, darkness for three. Let his pulse beat ever so 

 irregularly from toil and anxiety, he could discern by it infallibly, that the 

 dark interval was three, the light one, and thus that this was the cynosure to 

 lead him to the haven whence he would be. Xor could he mistake Fire 

 Island light for that at Sandy Hook, for it would signal twenty-two, first 

 two, next two but never one. Prof. JBache, Address before the American 

 Institute, 1856. 



NEW METHOD OF DETERMINING THE DENSITY OF THE EARTH. 



At the Albany Meeting of the American Association, Prof. Alexander 

 remarked that the celebrated experiment of determining the density of the 

 earth by the deflection of a plumb line near a mountain, was open to many 

 objections, arising from the imperfections of the measurement of the density 

 of the mountain, and of the deflection of the line ; and proposed the reverse 

 of Mohamed's doctrine, instead of carrying the plumb line to the mountain, 

 he would bring a mountain to the plumb line. He would build up by the 

 side of a plumb bob, on which bob he would attach the most delicate spirit 

 levels, a mass of lead or iron, in the form of a sphere twenty-five or thirty 

 feet in diameter, and observe the change of the spirit levels. 



Dr. Gould observed that this method was not equal to that of Cavendish, 

 since the torsion balance is a more delicate test of minute forces, such as 

 the attraction of the artificial mountain, than a spirit level can be. He also 

 repeated the suggestion of "\Yeber, that the Gaussian mirror be used, by which 

 a radius of unlimited length may be obtained. 



Prof. Bartlett thought there was a more fundamental objection to Prof. 

 Alexander's method; that the spirit level would only show its own level 

 surface, and that would be the resultant of the combined action of the earth 

 and the artificial mountain. 



