1837,] 



Observations at Moulmem. 



51 



in order to admit of comparison with other experiments, it ought further 

 to be determined to what portion of the whole heavens, and in what 

 direction, the instrument was exposed. It may be remarked that the 

 method of measuring the intensity of terrestrial radiation by the maxi- 

 mum difference between the shaded and exposed thermometers is sub- 

 ject even to greater error than in the case of solar radiation, for it is 

 evident, from the extremely heavy dew on a clear night, that the whole 

 surface of the ground attains the temperature of the dew-point, and 

 hence a detached thermometer must sink below it. But as soon as this 

 happens, the decrement of temperature can no longer be a measure of 

 the intensity of radiation in the same sense it was before, for there is 

 now a continual influx of heat to the bulb from the condensing vapour. 

 It might easily be shewn by the same process of reasoning employed 

 to establish, the dew-point formula, that if the tension of vapour cor- 

 responding to the minimum temperature of the exposed thermometer, 

 be subtracted from the tension of vapour at the dew-point, and the- dif- 

 ference- be multiplied by 87, this product will express the additional 

 number of degrees, on Fahrenheit's scale, which the exposed thermo- 

 meter would sink, were it not for the dew. 



P. S.— -I venture to add, in a postscript, a few remarks which, while 

 they are not altogether foreign to the subject of this paper, are not 

 worthy of a separate place. 



It is sometimes required to determine the internal diameter of a 

 barometer tube when circumstances prevent its direct measurement, 

 As this method of doing so, though easily derived from an elementary 

 optical proposition, may not immediately suggest itself to those having 

 only accidental occasion for it, it is here annexed. Apply the points 

 of a pair of compasses to the external surface of the tube, and holding 

 the eye opposite to them, and as far removed as is consistent with dis- 

 tinct vision, measure the least distance between the parallel sides of 

 the bore. Two-thirds of this quantity is nearly equal to the true in- 

 ternal diameter. A nearer value is obtained by the following rule : 

 add to the external diameter of the tube twice the distance of the eye 

 from one of the points of the compasses, and divide by twice the dis- 

 tance of the eye ; the product of this quantity into the approximate 

 value already found, is very nearly equal to the internal diameter of the 

 tube. When the external diameter is .333 of an inch, the apparent 

 internal diameter .12 of an inch, and the distance of the eye 4 inches, 

 the results of the two foregoing approximations, and of an exact calcu- 

 lation give, respectively, as the values of the internal diameter, ,03 oi' 

 an inch, .0833 and .08367. 



