174 ANNUAL OP SCIENTIFIC DISCOVERY. 



in the equator. The conclusion therefore follows that the equatorial 

 regions of the sun are hotter than the polar. M. Secchi's observations 

 did not extend to the spots of the sun ; yet in a few trials they were 

 found to produce a sensible diminution of temperature. He says that 

 the prevalence of the spots about the equatorial region corresponds 

 well with the view that this part is the hottest in the sun. 



NEW MODE OF MEASURING HIGH TEMPERATURES. 



THE pyrometers of Wedgewood and Daniell, have both been found 

 inapplicable to measure readily the high temperatures of furnaces and 

 melted metals, as they are not easily obtained. A very ingenious 

 method of doing this, and within the means of every one, has been 

 proposed by Mr. John Wilson. According to his plan, a given weight 

 of platinum is exposed for a few minutes to the fire, the temperature 

 of which is required to be measured, and then plunged into water of 

 a determined weight and temperature. The number of degrees of 

 heat, measured by a small thermometer in the liquid, thus communi- 

 cated by the heated platinum to the liquid, becomes the index to the 

 actual temperature of the fire. 



In practice, the quantity of water by weight may be just double 

 that of the platinum, and the amount of heat indicated in this case for 

 each degree that the temperature of the water is raised, is readily 

 found by direct experiment, and when once found, we have the con- 

 stant value of 1 of this pyrometer. The weight of water being to 

 that of the platinum as 2 to 1, each degree will equal 31^ Fahr. 



To obtain accurate results, one must allow for the heat absorbed 

 by 



1st. The mercury of the thermometer in the water. 



2d. The glass bulb and stem of the thermometer. 



3d. The vessel containing the water. 



4th. The heat retained by the piece of platinum. 



The portion of total heat absorbed by the several bodies compared 

 to the portion received by the water will be in proportion to their sev- 

 eral weights and the specific heat of each compared with water. 

 If the platinum weigh 1,000 grains, the tinned iron vessel, 3 inches 

 diameter and 2 inches deep, 658 grains, the mercury in the thermom- 

 eter 200 grains, and the glass bulb and stem immersed 35 grains, then, 

 since the platinum compared with an equal weight of water will only 

 have to absorb 1-32 part of the actual heat that water will, in order 

 that both may exhibit the same temperature, iron only 1-9, mercury 

 1-30, and glass 1-6, the correction is thus obtained: 



Equivalent grains 



of water. 



Platinum 1,000 grains X 1-32 specific heat = 31 

 Iron 658 " X 1-9 " " = 73 



Mercury 200 " X 1-30 " " = 7 

 Glass 35 " X 1- G " " 6 



Total, 1 1 7 



