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 supposed 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 calculated 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 another, to the precision of about three minutes. LENGTH OF DAYS AT THE POLES. During the summer, in the northern hemisphere, 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. Indeed, at the Poles there is but _one day and one night_; for the sun shines for six months together on one Pole, and the other six months on the other Pole. HOW THE DISTANCE OF THE SUN IS ASCERTAINED BY THE YARD-MEASURE. Professor Airy, in his _Six Lectures on Astronomy_, gives a masterly analysis of a problem of considerable intricacy, viz. the determination of the parallax of the sun, and consequently of his distance, by observations of the transit of Venus, the connecting link between measures upon the earth’s surface and the dimensions of our system. The further step of investigating the parallax, and consequently the distance of the fixed stars (where that is practicable), is also elucidated; and the author, with evident satisfaction, thus sums up the several steps: