NATURAL PHILOSOPHY. 165 



serrations were made in varied forms, each one being continued for 104 hours, 

 and the correspondence between them was so exact as to leave no doubt of 

 the accuracy of the experiments. The results showed a difference of two 

 seconds and a quarter in 24 hours between the pendulum at the lower point 

 and that at the mouth of the pit, the lower one having gained that much in 

 consequence of the greater attraction of gravitation. Professor Airy said it 

 could be mathematically demonstrated that the counteracting attraction of the 

 shell of the earth above the pendulum would be exactly neutralized by the 

 attraction of the same thickness of matter in other parts of the sphere, so that 

 the actual force of gravitation acting on the pendulum would be represented 

 by the mass beneath, not including the shell, of 1600 feet. All the requisite 

 computations have not yet been made, but sufficient has been done to ascer- 

 tain that the mean density of the earth is greater than the estimated density 

 by the Schechallian experiment. By that experiment it appeared that the 

 mean density is between five and six times the specific gravity of water, 

 while the pendulum experiments make it between sis and seven times more 

 dense. 



Pendulum Detachment. An ingenious and novel pendulum detachment for 

 church clocks has been invented by J. E. Brown, of Providence, R. I. This 

 arrangement, illustrated with diagrams, may be found described in the Scien- 

 tific American, February 10th, 1855. 



OX SOLAR REFRACTION. 



Among other interesting and important consequences of the dynamical 

 theory of heat, Professor "W. Thomson having deduced the necessity of a re- 

 sisting medium, the condensation of this about the sun, and a consequent re- 

 fraction of the stars seen in that neighborhood. Professor Piazzi Smyth has 

 endeavored to ascertain, by direct astronomical observation, whether any such 

 effect was sensible to our best instruments. Owing to atmospheric obstruc- ' 

 tions, only three observations, yielding two results, had been yet obtained ; 

 but both these indicated a sensible amount of solar refraction. Should this 

 effect be confirmed by more numerous observations, it must have important 

 bearings on every branch of astronomy ; and as the atmosphere at all ordinary 

 observatories presents almost insuperable obstacles, the author pointed out 

 the advantage of stationing a telescope for this purpose on the summit of a 

 high mountain. Prov. British Association. 



COLOR BLINDNESS. 



This term is applied to an inability to distinguish different colors. It in- 

 cludes all varieties and degrees of the affliction. In some cases there is total 

 blindness to colors, the distinction of black and white alone being perceived. 

 More frequently there is inability to discern a single color, such as red, or in- 

 ability to distinguish between two colors, such as red and green. A work on 

 this subject has recently been published by Dr. George "Wilson, of England, 

 in which he takes occasion to point out the danger attending the present sya- 



