1895." 



the Radiant Heat from the Moon during an Eclipse, 635 



of a fixed coil carrying the current to be measured, causing the motion 

 of a magnetic needle delicately suspended within it, there is a fixed 

 horseshoe magnet, between whose poles is suspended a coil which the 

 current enters from, say above, and leaves below, through two fine 

 wires, which hold it in position. To this instrument there is the 

 drawback that the wires from tlie coil must be small, otherwise they 

 would impede its movements. Hence it is unfitted for the measure- 

 ments of currents of low tension like those coming from the thermo- 

 couple. 



Mercury contacts, owing to capillary forces and an incipient 

 coating of oxide, give endless trouble. Mr. Boys has modified the 



FIRST qUARTER 

 LUNAR HEAT f W VERY 



Fig. 15. 



instrument by rigidly connecting the thermocouple with the coil, or 

 the single circuit which rej3laces the coil. Thus a far more sensitive 

 arrangement than mine (some 6000 times he states) has become pos- 

 sible ; but it has the disadvantage that it cannot be fitted to the 

 ordinary telescope, as it must never change its position, and hence a 

 siderostat — a not very simple thing to provide when we work with 

 large apertures — is essential. I have a radiomicrometer, but 1 have 

 not as yet found an opportunity for the construction of the required 

 siderostat.* 



Slide 16 gives | Mr. Boys' curve for the deviation as the Moon's 

 image slowly passes by diurnal motion over the instrument, thus 

 showing approximately the distribution of the radiation from the 

 lunar disc. Owing to the very small mass of metal to be heated, 

 and the absence of swing, or the " dead beat " of the needle, this 

 method of observation has become possible. With our arrangement 

 the comparative slowness of the needle in coming to rest has entirely 

 prevented this method of observation. 



* See Phil. Trans, vol. clxxx. 1890, p. 159. 

 t See Proc. Roy. Soc. vol. xlvii. 1890, p. 480. 



