200 On Solar Radiation. 



might be obtained with much exactness, and thus a highly 

 valuable datum secured to science. The observers would, 

 of course, agree to make their observations strictly simultane- 

 ous, and should, therefore, compare watches before parting. 



" The actinometer is also well calculated for measuring the 

 defalcation of heat during any considerable eclipse of the 

 sun, and the Committee would point out this as an object 

 worthy of attention, as many eclipses, invisible or insigni- 

 ficant in one locality, are great, or even total in others. The 

 observations should commence an hour at least before the 

 eclipse begins, and be continued an hour beyond its ter- 

 mination, and the series should be uninterrupted, leaving to 

 others to watch the phases of the eclipse. The atmosphe- 

 ric circumstances should be most carefully noted during the 

 whole series." 



The intention of this instrument is to measure the action 

 of the sun's rays during a given portion of time. After the 

 instrument has been exposed for a short time to the sun, 

 and it is then shaded for one minute, the temperature of the 

 instrument decreases, as is shewn by the descent of the fluid. 

 At the end of the minute, if the instrument was still shaded, 

 the temperature would still further decrease ; but if the sun's 

 rays are then allowed to act upon it for one minute, the tem- 

 perature again increases ; therefore the whole action of the 

 sun's rays during the minute is not only measured by the in- 

 crease of temperature in that time, but also by the decrease 

 of temperature while their action was being arrested, and 

 which would have had effect had the instrument been 

 shaded. Therefore the action of the sun's rays is measured 

 by the mean cooling during the minute, added to the heat- 

 ing power during that time, or by the mean heating power 

 added to the measure of the cooling. It is very necessary 

 to attend to this explanation of the action of the instrument, 

 for I find that some observers have been led into error by 

 mistaking the principle, and among others, a valuable set of 



