228 



Lieut. Hennessey on Actinometrical [Dec. 22, 



groups of 5 aud 4 sun-observations respectively (those in shade are here 

 understood), the fifth sun being common to both groups. 



Nine ditto, ditto, gave 5 and 5. 



Ten ditto, ditto, gave 6 and 5. 



Eleven ditto, ditto, gave 5, 4, and 4 ; and so on. 



(9) The mean results by each group were next all corrected for excess 

 of temperature above 32° Fahr., the Table of expansion for alcohol by 

 Kopp, given in Gmelin's ( Chemistry,' being employed for this purpose. 

 After this step the results by A were entered in the result-abstract Table 

 and the corresponding values, in terms of A glass off, found by means of 

 factor No. 1. The results by B were further corrected in the record of 

 observations by means of factor No. 3. Being now in terms of A glass on, 

 they were introduced into the result-abstract Table, and there reduced to 

 A glass off, by means of factor No. 1. 



(10) Thus the result- abstract Table contains the values obtained by 

 each actinometer expressed in terms of A glass on as well as glass off. 

 The latter values are those made use of in projecting the actinometric 

 curves, and in the discussion of the observations. The former values will 

 be useful should the Royal Society see fit to send me a third actinometer 

 whose constant for reduction to the Kew standard has been duly ascer- 

 tained. At present the required relation is wanting ; for though Professor 

 Stokes was so exceedingly kind as to visit Kew with the object of getting 

 the actinometers A and B verified, the necessary observations could not be 

 made from want of time. 



(11) Turning, now, to the diagram of actinometric curves and to the re- 

 sult-abstract Table, it is readily seen that the solar radiation decreases 

 from some time about apparent noon both towards sunrise and sunset. This 

 hour-angle change is least perceptible for some + 1 hour (or less) from 

 noon — a condition which indicates that observations for relative or abso- 

 lute intensity are most valuable when made during this interval. Indeed 

 even desultory observations might acquire importance by being restricted 

 to these hours, the absence of cloud, mist, haze, or other abnormal inter- 

 position being always supposed. 



(12) But besides the hour-angle change, the intensity is liable to rises 

 and falls brought about in only a few minutes of time. Any observer who 

 has used the instrument could venture to affirm that these fluctuations are 

 not due to fallibility of observation. Whether their magnitude varies with 

 that of the intensity or otherwise may be a matter of interest to ascertain ; 

 and to this end series of observations, continued for as long a period as the 

 construction of the instruments will permit, appear desirable. 



(13) Again, there is a change of intensity from day to day, apparently 

 not due to alteration in the sun's declination, so that the average daily 

 curve (about noon) is higher or lower without any visible reason. It is 

 interesting to notice that this daily change was common to Mussoorie and 



