NATURAL PHILOSOPHY. 



161 



zodiacal light appears to reach nearly to the earth at present ; and in past time 

 it may be that the earth has been in a dense enough part of it to be kept hot, 

 as the sun is now, by drawing in meteors to its surface. This calculation is 

 according to Mr. Waterson's form of the theory, but, according to Prof. 

 Thompson's, the fall of meteors must be twice that determined above. Then 

 the whole surface would be covered annually to the depth of sixty/ feet, and 

 the sun would grow in diameter a mile in eighty-eight years. Even at this 

 rate, it would take 4,000 years to grow sufficiently to make the change appa- 

 rent to the most refined observations. 



A body of such dimensions as the sun might, by entering a cloud of 

 meteors, become incandescent intensely in a few seconds, and on again 

 getting to a position comparatively free from meteors, as suddenly become 

 dark again. 



If the sun is burning, -and its conditions are similar to those of the earth, 

 the fire would be choked, and by no conceivable adaptation of air and fuel, 

 could keep a light for more than a few minutes. If it contains within 

 itself all the elements of combustion, to give the amount of light and heat 

 required would by demonstration cause it to burn away in 8,000 years. If 

 the sun has been burning at that rate, he must have been of double diameter, 

 quadruple heating-power, and eight fold mass, only 8,000 years ago. 



OX A THERMOMETER FOR MEASURING FLUCTUATIONS IN 



TEMPERATURE. 



The following description of a new thermometer invented by Mr. Stewart, 

 was described by Mr. Welsh at the last Meeting of the British Association : 

 If a bulb be blown between two thermometric glass tubes of unequal bores, 

 and the instrument be filled with mercury in the same manner as an ordinary 

 thermometer, and laid horizontal or nearly so, it will be found that contrac- 

 tions from cold take place only in the narrow bore, and expansions from 

 heat only in the wide one. The reason of this seems to be, that while the 

 temperature remains the same the mercury is kept at rest, and prevented from 

 retreating from the small bore into the bulb by friction ; but, when a motive 

 force is supplied by a change of temperature, the motion of the mercury takes 

 place in that direction in which it is most aided by capillary action. It was 

 suggested by Mr. Welsh to the author, that such an instrument might be used 

 to measure fluctuations of temperature. And the author thinks it might be 

 applied to measure with exactness the power of a source of radiant heat ; for 

 by alternately interposing a screen between this instrument and the source of 

 heat, and withdrawing the same screen, the effect of the source on the 

 mercury would be multiplied by the number of times this operation was 

 performed. In constructing such an instrument care must be taken that the 

 tubes used are quite free from dirt or moisture, and that they are not bent, 

 but form one straight line, the bulb being in the middle, and swelling out 

 symmetrically from both its extremities. The best proportion between the 

 capacities of the bores is perhaps about 1 to 4, and the best arrangement of 

 bores seems to be one suggested by Mr. "Welsh, viz. a round bore for the wide 



