SOLAR ECLIPSE, MAY (i, 1883, 73 



mercurial thermometer in the instrument shelter, reduced to centigrade measures. In a few cnses 

 tlie values were obtained by interpolation. 



The column of differences [a — b) shows the discrepancies between tln^ two methods. The signs 

 are + for ruorniug and evening values, and — for those in the middle of the day, as should be the 

 case by taking a constant value of c. A comparison of the two methods shows no reason for 

 jtreferring one to the other. The one which introduces the value of c — equation (2) — is open to the 

 objection that it assumes a constant value for c which it has been shown is only approximately true ; 

 it is possible that the deviations from constancy are due to the circumstances of these particular 

 observations, and that consequently the intensities are not strictly accurate as computed. On the 

 other hand, the method which uses the shade temperatures — equation (1) — is open to the uncertain- 

 ties in the observations of that quantity, and assumes that these temperatures may be adopted as 

 the temperatures of the inclosure of the black bulb thermometer. There seemed to be no reason 

 for giving preference to either of the reductions. The last column contains the 7nean of the two 

 series of values, which are adopted as the solar intensities given by the conjugate thermometers. 



2. RKDVCXION OF OBSERYATIOIfS MADE WITH VIOLLE'S BULBS. 



The theory of these instruments is given by the inventor in his work upon solar radiation, 

 above referred to, pp. 17-22*. According to the method there explained, the intensities may be 

 obtained bv the equation: 



M U' 



iu which 



M = the difference between the reading of the thermometer in the blackened sphere and the 

 air temperature. 

 . «' = the difference between the reading of the thermometer in the gilded sphere and the air 

 temperature. 

 a = the absorbing power of the gilded sphere in terms of that of the blackened sphere. 

 K = a constant depending upon the instrument, and found by the methods explained, 

 if K is undetermined, relative intensities can be obtained by the equation: 



uu' 

 1 = u'— au 



. In this equation the quantities u and n' are obtained directly from the observations, but the 

 constant a must be derived experimentally. After the return of the expedition, a number of 

 observations of this constant were made by screening the spheres and ol)serving the rate of cooling 

 of each, and also by observing the rate of increase of temperature when full sunlight was allowed to 

 fall upon them after they had assumed the shade temperature behind the screen. The experiments 

 gave the value a =0.5, the large value being due to the tarnishing of the gilt, which was caused 

 in ])artby the effect of the sea voyage. It is in part also apparently due to the poor manner in 

 which the gilding has been done, as the same deterioration has been found in instruments which 

 have never been used in actual work. Similar observations with a pair of instruments in which 

 there was a good polisli on the gilded sphere gave the value a = 0.3. It was decided to use the 

 value 0.1 in the reduction as probably representing nearly the correct value for the condition 

 of the gilt at Caroline Island. An approximate value is sufficient for determining the relative 

 intensities, which is the aim of the present investigation. 



The following table contains the computation for the same dates aiul times at which the 

 readings of the conjugate thermometers were reduced. Other observations were reduced, but 

 it is thought unnecessary to give the computation, as they sliow discordances due to the effect 

 of passing clouds to a greater degree than those here given. 



* See also Annales de Chimie et de Physique, 5= s6rie, t. XVII, 187t>. 

 S. Mis. 110 10 



