154 ANNUAL OF SCIENTIFIC DISCOVERT. 



DURATION OF THE SUN'S HEAT. 



Profr>s<5or Thomson assigns to the sun's heat, snpposinj]^ it to be 

 maintained by the ajjpulse of masses of matter, a limit of 300,000 

 years ; and to the period of coolinj^ of the earth, from universal 

 fusion to its actual state, 98,000,000 years. These are the lowest 

 estimates sanctioned by any mathematician. 



DIFFERENTIAL RHEOMETER. 



Tn a note, on the em]iloyment of a double-wire rheometer in 

 experiments on radiant heat, sent to the Academy of Sciences by 

 M. P. Desains, the author states that he employs a kind of differ- 

 ential apparatus essentially comjiosed of a sino^le source of heat, 

 of two ])iles, of a doul)le-wire rheometer, and finally of a rheo- 

 stat. The apjiaratus is so arran<^ed that the equilibrium, once 

 ol)tained, remains uniform however the heat from the source 

 varies ; but if the smallest variation takes place in one of the 

 radiations, the needle quits the zero point. M. Desains has ap- 

 plied this api)aratus to the examination of the absorption of heat 

 by transparent gases, and finds that it gives very delicate and 

 certain indications. — Scientific American. 



CONDUCTING POWER OF MERCURY. 



M. Gripon has presented a note to the Academy of Sciences on 

 the conducting power of mercury for heat. Experiments made 

 after Peclet's method showed that, if tlie conducting power of 

 silver equals 100, that of mercury equals 3.54. It stands, there- 

 fore, the last of the metals, and a little before marble and gas 

 coke. The author mentions that, in this case, the conducting 

 power for heat and for electricity are very difi'erent, the former 

 being 3.54, the latter 1.80. — Mechanics' Magazine. 



A NEW PYROMETER. 



Messrs. St. Claire Deville and Trooste have invented a pyrome- 

 ter capable of measuring a temperature reaching as high as 1530° 

 C. At this heat, the inventors state, co^Dper and silver are va- 

 porized, and feldspar perfectly fused. 



IMPBOYEMENT OF THE HYPSOMETER. 



The boiling points of fluids depend on the pressure of the air ; 

 the greater the altitude of any place above the ordinaiy level of 

 the earth's surface, the lower the boiling point of a given fluid, 

 because the less the barometric pressure in that place ; and hence 

 the height of any place may be found by means of the boiling 

 point of water in that place. With this object, a peculiar kind of 

 thei-mometer, termed a hypsometer, or, more con-ectly, a hypso- 

 thermometer, has been constructed. It is marked not with degi'ees 

 of temperature, but with barometric pressure corresponding to these 



