to the Antarctic Regions. 21 3 



beginning and ending with the same ; though it would be more con- 

 veniently done by two observers observing simultaneously at the 

 same place, and each registering his own instrument. An hour or 

 two thus devoted to comparisons in a calm clear day, and under easy 

 circumstances, will in all cases be extremely well bestowed. 



Neither should each observer neglect to determine for himself the 

 heat stopped by each of his glasses. This may be done also by al- 

 ternating triplets of observation made with the glass on and off, thus, 



beginning and ending with the glass off, and (as in all cases) begin- 

 ing and ending each triplet with a sun observation. For the pur- 

 pose now in question a very calm day must be chosen, and a great 

 many triplets must be taken in succession. It will be found that a 

 single thickness of the ordinary bluish or greenish plate glass stops 

 about 0*20 (=3") of the incident calorific rays ; a second glass 

 about 0*16 (or a materially less proportion) of those which have 

 escaped the action of the first. No two glasses, however, are pre- 

 cisely alike in this respect. 



Very interesting observations may be made by two observers fur- 

 nished with well-compared actinometers, the one stationed at the sum- 

 mit, the other at the foot of some great elevation, especially if the 

 stations can be so selected that the observers shall be nearly in the 

 line of the incident sunbeam at the time of observation, so as both 

 to lie in the atmospheric column traversed by the rays. Many con- 

 venient stations of this kind might be found in mountainous coun- 

 tries ; and by repeating the observation two or three times under fa- 

 vorable circumstances, interchanging observers and instruments, 

 Slc, and accompanying the observations with all circumstantial and 

 local elements of precision, there is no doubt that the co-efficient of 

 extinction of solar heat in traversing at least the lower strata of our 

 atmosphere 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 simultaneous, and should, 

 therefore, compare watches before parting. 



The actinometer is also well calculated for measuring the defalca- 

 tion of heat during any considerable eclipse of the sun, and the Coun- 

 cil would point out this as an object worthy of attention, both 

 at the fixed stations and on board the vessels ; as many eclipses in- 

 visible or insignificant 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 termina- 

 tion, and the series should be uninterrupted, leaving to others to 

 watch the phases of the eclipse. The atmospheric circumstances 

 should be most carefully noted during the whole series. 



