BoEDDiCKER — On Lunar Radiant Heat. 4S5 



Observatory during the whole of its course. Such an opportunity occurred on the 

 28th January, 1888, and was again exceptionally favoured by the weather, the 

 sky being perfectly clear (without any wind) the whole night through. 



2. The mode of observing was essentially the same as in 1884. The 

 .thermopiles of 1888, however — single thermic junctions — had been newly made 



by Lord Rosse for the occasion, and were very considerably more sensitive than 

 the former ones. Consequently, one important alteration could be made, namely, 

 each pile was exposed for 30 seconds sidereal time only (against one minute in 

 1884); or, in other words, the galvanometer was read off every half minute, as 

 this time sufficed to obtain the maximum deflection. The labour of observing 

 was divided in such a way that Lord Rosse watched the thermopiles, and the 

 driving clock of the telescope, while I took all the readings of the galvanometer. 

 The alternate exposing of the thermopiles, which is readily done by simply 

 raising and lowering the telescope (of three-feet aperture) was done by an 

 assistant, the signal for it being given by me every 30 seconds by means of an 

 electric bell. The reading off of the galvanometer was carried on as far as possible 

 continuously during the whole of the ecliiDse. Some interruptions could, however, 

 not be avoided, and may be here at once enumerated. 



3. Preliminary observations were taken from 3'' 20'2" to S*" 25-2", and from 

 ^h g.i^m ^Q ^h u.^m gi£[gi.gal time. The continuous series commenced at 4'' IG-?"". 

 No observations were obtained from: — 4'' 37'2'" to 4'' 39 ■2" (the zero point of the 

 galvanometer had to be changed) ; b^ 6'2" to 7"7™ (three readings were missed, the 

 signal not being understood by the assistant at the telescope) ; 5'' 14-2" to 20"2'" 

 (the clock-sector had to be wound back); 5'' 38"2"' to 38"7" (one reading was 

 missed); 5"^ 48-7"' to 6^ 0-2'°; 6" 15-7" to 22-7"; 6'' 25-7" to 45-7" (the driving- 

 clock stopped through the slipping off of a band). From 6" 45-7" till 8'' 23-2'^, 

 which period embraces the time from 16" 3"" after the beginning, till 15'6" after 

 the end of totality, a number of galvanometer readings were taken. Since, 

 however, it became practically impossible to make sure of the lunar image being 

 concentrated on the thermopiles (owing to the extreme faintness of the eclipsed 

 moon), these observations, though given further on, could not be utilized for 

 the construction of the final heat-curve. Observations began again at 8'' 28'2°', 

 and lasted till ll"^ 52-7°', with the following interruptions :— 9"^ 18-2" to 23-2'^ 

 (the clock-sector had to be wound back); 9'' 26-7" to 27"7°' (one reading was 

 missed); 9'' 50-7"" to 52-2"' (two readings were missed); lO'^ 49-2'" to IP 19-2°^ 

 (observing was stopped for thirty minutes). It will be perceived from the above 

 that observing was practically carried on from 7^ 19"" till 15'' 45"" mean time 

 Grreenwich, during which period I read off the galvanometer altogether 638 

 times. 



