ON SOLAR EADIATION. 33 



artificial sonrce of heat, such as a fire or a stove, the intensity of which 

 might likewise be made to vary. By this means the conditions of the 

 instrument when facing the sun might be fairly represented. 



Experiments of this nature were made at Manchester by Mr. Shep- 

 herd, acting under the superintendence of Professor Stewart, and these 

 were reduced by Professor Stokes. 



It was ascertained from these experiments that the internal thermo- 

 meter represented with great exactness the temperature of the cube 

 such as it was 3^ minutes before ; in other words, there was a lagging 

 time of the internal thermometer equal to 3^ minutes. 



"We may thus find what woald be the reading of the internal thermo- 

 meter if the balance were perfect between the gain of heat by direct 

 solar radiation and the loss of heat by communication to the environ- 

 ment ; and as the latter is approximately proportional to the difference 

 of temperature of the envelope and internal thermometer, and the devia- 

 tion from exact proportionality admits of determination by laboratory 

 experiments, w-e have the means of measuring the former. We must 

 bear in mind that the lagging time of the final thermometer may be 

 different from that of the thermometer with which these experiments 

 were made. 



It was likewise ascertained that the difference between the tempera- 

 ture of the internal thermometer and that of the case need not exceed 

 20° Fahr., and that a comparatively small lens and hole would suffice for 

 obtaining this result. 



In consequence of this preliminary information, we have made the 

 following additions to the instrument described in our last report : — 



(1) We have had it swung like the ordinary actinometers with a mo- 

 tion in altitude and azimuth, and with two moderately delicate adjusting- 

 screws, one for azimuth and another for altitude adjustments. 



(2) We have had a thermometer cenfcrically placed in the interior. 

 Tlie graduation of the stem is very delicate, and extends from 20° to 

 120° Fahr., the reading being taken from one of the sides. The bulb is 

 of green flint, and the stem of colourless glass. 



(3) We have also had a small plate of quartz cut and polished and 

 mounted so as to cover the hole, and to be easily removed and replaced. 

 The object of the plate is to prevent irregularities arising from irregular 

 issue of heated air through the hole, entrance of cooler air blown in by 

 wind, &c., and the choice of material was influenced by the wish to per- 

 mit of frequent cleaning without risk of alteration by scratching. 



We ought to mention that as it would be difficult to procure the loan 

 of a good heliostat, and expensive to make one, we resolved that in the 

 preliminary experiments the adjustments to keep the sun's image on the 

 hole should be made by the observer. Hence the necessity for the 

 adjusting-screws already described. 



The Committee have expended £18 10s., and return to the Associa- 

 tion a balance of £1 10s. 



They suggest that they should be reappointed, and that the sum of £10 

 be placed at their disposal to defray the expense of further experiments 

 connected with the instrument. 



18S7. 



