S Curiosities of Science. 65 



mass, upon Terrestrial Magnetism, is confirmed by Sabine in 

 the ingenious observation, that the period at which the inten- 

 sity of the magnetic force is greatest, and the direction of the 

 needle most near to the vertical line, falls in both hemispheres 

 between the months of October and February ; that is to say, 

 precisely at the time when the earth is nearest to the sun, and 

 moves in its orbit with the greatest velocity. 



IS THE HEAT OF THE SUN DECREASING ? 



The Heat of the Sun is dissipated and lost by radiation, and 

 must be progressively diminished unless its thermal energy be 

 supplied. According to the measurements of M. Pouillet, the 

 quantity of heat given out by the sun in a year is equal to that 

 which would be produced by the combustion of a stratum of 

 coal seventeen miles in thickness ; and if the sun*s capacity for 

 heat be assumed equal to that of water, and the heat be sup- 

 posed drawn uniformly from its entire mass, its temperature 

 would thereby undergo a diminution of 20 '4 Fahr. annually. 

 On the other hand, there is a vast store of force in our system 

 capable of conversion into heat. If, as is indicated by the 

 small density of the sun, and by other circumstances, that 

 body has not yet reached the condition of incompressibility, 

 we have in the future approximation of its parts a fund of 

 heat, probably quite large enough to supply the wants of the 

 human family to the end of its sojourn here. It has been cal- 

 culated that an amount of condensation which would diminish 

 the diameter of the sun by only the ten-thousandth part, would 

 suffice to restore the heat emitted in 2000 years. 



UNIVERSAL SUN-DIAL. 



Mr. Sharp, of Dublin, exhibited to the British Association 

 in 1849 a Dial, consisting of a cylinder set to the day of the 

 month, and then elevated to the latitude. A thin plane of 

 metal, in the direction of its axis, is then turned by a milled 

 head below it till the shadow is a minimum, when a dial on 

 the top shows the hours by one hand, and the minutes by an- 

 other, to the precision of about three minutes. 



LENGTH OF DAYS AT THE POLES. 



During the summer, in the northern hemisphere y places 

 near the North Pole are in continual sunlight the sun never 

 sets to them ; while during that time places near the South 

 Pole never see the sun. When it is summer in the southern 

 hemisphere, and the sun shines on the South Pole without 

 setting, the North Pole is entirely deprived of his light. In- 

 deed, at the Poles there is bit one day and one night ; for the 



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