S. P. Langley — Temperature of the Moon. 431 



The bolometer must remain unmoved, and the direction of the 

 heat-receiving apparatus to the east or west of the moon must 

 always be understood to be obtained by a slight motion of the 

 siderostat mirror. 



That the minute change in the angle of presentation of the 

 face of this mirror does not affect its own radiations apprecia- 

 bly, might well be anticipated, but is a fact which has not been 

 left unproven by direct experiment. The bolometer, then, en- 

 closed in a non-conducting case which cuts off radiations from 

 every object but the mirror or prism immediately in front of 

 it, practically feels only the radiations from the moon, or from 

 the sky immediately on each side of it, except when the screen 

 is interposed. Mirrors and prisms do indeed radiate heat to it 

 from their own substance, but these radiations may be consid- 

 ered as absolutely constant, and as therefore absolutely negligi- 

 ble during the brief cycle of a single observation. 



"We confine ourselves here to the above general explanation, 

 referring the reader who may be interested in the details of 

 the observations to the original memoir, remarking, however, 

 that the actual spectral position of a ray is given by a circle ; 

 reading to 10" of arc, and that previous measures of our own* 

 enable us to convert this arc into wave-length. This fixes the 

 position of bands in the spectrum. The amount of heat in any 

 portion of the spectrum is, within the narrow limits of errors 

 of observation, strictly proportional to the deflection on the gal- 

 vanometer scale (the conditions of the bolometer, battery cur- 

 rent, galvanometer, etc. remaining constant). As these degrees 

 are arbitrary, they are converted into thermometric degrees by 

 a process fully detailed in the original memoir. 



The preliminary record of the humidity, state of the sky, 

 temperature, etc., is nearly self-explanatory. We need only ex- 

 plain that " Rock-salt lenses at 37 cm " refers to the fact that the 

 focal length of these lenses increases from 35 0m in the visible 

 spectrum to one indefinitely greater with heat of great wave- 

 length, and that this focus accordingly needs to be adjusted for 

 the particular part of the heat spectrum under study. 



"Deflection per degree Centigrade" refers to the use of a 

 constant determined for each evening, giving the actual deflec- 

 tion the galvanometer produces, for each degree of excess of 

 temperature, in a certain standard Leslie cube at a certain 

 standard distance from the bolometer. 



The order of observation consists first, in noting the time. 

 (We will suppose, in the example which follows, that the time 

 is 9h. 08 M. T., on the evening of February 9, 1887) ; next, in 

 noting the prismatic deviation corresponding to the actual posi- 

 tion of the bolometer in the spectrum, which in this particular 



* See this Journal, xxxii, August, 1886. 



