ON DIRECT INTENSITY OF SOLAR RADIATION. 29 



constructed as to be readily susceptible of solar influences. It is proposed 

 to make it of green glass (a good absorber and radiator), and to give it 

 a flattened surface in the direction perpendicular to the light from the hole. 



Such an instrument should be so adjusted as to receive the sun's b>ht 

 continuously through the hole, and the objects of record would be the 

 simultaneous heights of the two thermometers, the one givino- the tem- 

 perature of the enclosure, and the other of the central thermometer. There 

 are two conceivable methods by which the necessary adjustment with 

 regard to the sun's light might be secured, namely, («) the enclosure 

 might be subject to an equatorial motion so as to follow the sun, or (/3) 

 the enclosure might be kept at l-est and the solar rays kept upon the hole 

 by a heliostat. Captain Abney is of opinion that the latter arrangement 

 is, mechanically, much preferable to the former. 



As the direction of the earth's axis may be chosen as that into which 

 the sun's light is to be reflected, a heliostat of a very simple construction 

 will suffice ; and as the angle of incidence on the mirror of such a heliostat 

 changes only very slowly with the season, there is no difficulty in applying 

 the small correction required for the change in the intensity of the reflected 

 heat consequent on the change in the angle of incidence. It is assumed 

 that the mirror of the heliostat is a speculum. 



It has been remarked by General Strachey that some such instrument 

 as this now suggested, even if not made self-recording, would have the 

 advantage of giving an observation without the objectionable necessity of 

 putting the light on for a given time and then shutting it off, operations 

 only suitable for trained observers. We think that it would be desirable 

 to construct an enclosure with its two thermometers such as herein 

 recorded. In all probability the loan of a heliostat and of an actinometer 

 might be obtained. By aid of the heliostat the sun's light might be kept 

 continuously upon the hole of the enclosure. The two thermometers 

 would be read, and the results compared with the simultaneous reading 

 of an ordinary actinometer. By such means it is believed that the best 

 method of constructing such an instrument and observing with it might 

 be found. 



We would therefore ask for a continuance of our Committee, with the 

 sum of 30Z. to be placed at our disposal for the purpose herein specified. 



Report of the Committee, consisting of Professor G. Carey Foster, 

 Sir William Thomson, Prof.ssor Ayrton, Professor J. Perry, 

 Professor W. Gr. Adams, Lord Eayleigh, Professor Jenkin, Dr. 0. 

 J. Lodge, Dr. John Hopkinson, Dr. A. Muirhead, Mr. W. H. 

 Preece, Mr. Herbert Taylor, Professor Everett, Professor 

 Schuster, Dr. J. A. Fleming, Professor G. F. Fitzgerald, Mr. K. 

 T. Glazebrook (Secretary), Professor Chrystal, Mr. H. Tomlin- 

 son, and Professor W. Garnett, appointed for the purpose of 

 constructing and issuing practical Standards for use in Elec- 

 trical Measurements. 



The Committee report that during the year the construction and testing 

 of standards of electrical resistance has been proceeded with. The 

 coils of 10, 100, 1,000, and 10,000 B.A. units, mentioned in the last 

 Keport, have been compared with the standard unit coils. An account 



