21S 



Instructio7is for the Scientific Expedition 



Every series of actinoraeter observations should be accompanied 

 with notices in tlie column of remarks of the state of the wind and 

 skj^ generally, the approach of any cloud (as seen in the coloured 

 glass) near to the sun ; the barometer and thermometers, diy and 

 wet, should especially be read off more than once during the series, 

 if a long one, and, if kept up during several hours, hourly. The 

 times should be correct to the nearest minute, at least as serving to 

 calculate the sun's altitude ; but if this be taken (to the nearest 

 minute or two) with a pocket sextant, or even by a style and shadow, 

 frequently (at intervals of an hour or less) when the sun is rising or 

 setting, it will add much to the immediate interest of the observa- 

 tions. When the sun is near the horizon, its reflection from the sea, 

 or any neighbouring water, must be prevented from striking on the 

 instrument ; and similarly of snow in cold regions, or on great ele- 

 vations in alpine countries. 



Every actinometer should be provided with a spare glass, and all 

 the glasses should be marked with a diamond ; and it should always 

 be noted at the head of the column of remarks ; which glass is used, 

 as the co-efficient of reduction from the parts of the scale (which are 

 arbitrary) to parts of the unit of radiation varies with the glass used. 



In the case of the actinometers sent out with the Expedition and 

 to the fixed observatories, these co-efficients will be ascertained for 

 each instrument and for each glass, provided it be practicable to pro- 

 cure any observations of the sun in the interval before the sailing of 

 the ships ; but at all events, an approximate value of the parts of the 

 scale in actines will be given by measurement of the dimensions, 

 and the glasses as well as the cylinders and capillary stems of the in- 

 struments, if accidentally broken, should have their fragments care- 

 fully preserved and labeled. 



The unit of solar radiation to be adopted in the ultimate reduc- 

 tion of the actinometric observations is the actine, by which is under- 

 stood that intensity of solar radiation, which at a vertical incidence, 

 and supposing it wholly absorbed, would suffice to melt one mil- 

 lionth part of a metre in thickness, from the surface of a sheet of ice 

 horizontally exposed to its action per minute of mean solar time ; 

 but it will be well to reserve the reduction of the radiations as ex- 

 pressed in parts of the scale to their values in terms of their unit 

 until the final discussion of the observations. 



Meanwhile, no opportunities should be lost of comparing together 

 the indications of different actinometers under similar and favourable 

 circumstances, so as to establish a correspondence of scales, Avhich 

 in case of accident happening to one of the instruments, will pre- 

 serve its registered observations from loss. 



The comparison of two actinometers may be executed by one ob- 

 server using alternately each of the two instruments, thus, 



