NO. 4 SOLAR RADIATION — ABBOT, FOWLE, AND ALDRICH 37 



instrument must tend to keep it at air temperature, and will diminish the 

 effect of the sun's rays. During calibration, steady, artificial, vertical air cur- 

 rents, of i to io meters per second, should be made to impinge upon the face 

 of the instrument, and the results tabulated in comparison with the record of a 

 standard instrument, not thus affected. It is partly on account of this strong 

 downward air current that I do not approve of your shallow cup, becafcse this 

 construction allows nearly free access of air currents to the heated surface, 

 which is liable to work great harm to the observations unless corrections are 

 determined from elaborate researches. ... I like the principle of the Violle 

 actinometer, namely, that of a wide, encompassing jacket at constant tempera- 

 ture ; and although some sort of a compromise must be made in your case, it 

 might be better to use a broader disk (even though this diminishes the sensi- 

 tiveness of the arrangement) and to place this disk at the center of a double- 

 walled alcohol jacket several inches in diameter. This will surely diminish 

 the wind effect, although I should still want to calibrate the thing with the 

 same strong downward currents as noted above. . . . By rights the temperature 

 of the alcohol jacket should be recorded, as in Violle's instrument. This 

 would require another thermometer, and a duplicate registering apparatus. 

 With an alcohol jacket the mercury thermometer would work down to nearly 

 — 40 centigrade, and, with the greater protection of a circumscribed aperture 

 and partial shielding from the wind, I should suppose that the apparatus might 

 continue to register when the outside air is quite a little colder than this. But 

 here I am only guessing, and there is the same objection to doing that in the 

 present case as there is to answering your " six questions." I prefer to leave 

 the guessing to you, and only say : Try it ! And I wish you success. 



In view of Mr. Very's excellent suggestions, four of the instru- 

 ments were arranged to be used by day, and one, with a row of 

 electric lights above the thermometer for recording purposes, was 

 arranged to be sent up at night. In two of the day instruments the 

 proposed electric heating was dispensed with. In place of it, there 

 was substituted a chamber of water (/, fig. 5), completely enclosing 

 the sides and bottom of the aluminum cup, within which is placed the 

 aluminum disk. A large number of copper strips for conducting 

 heat were disposed in all directions through the water chamber, and 

 soldered to the inside wall of it, so as to bring the water in intimate 

 thermal conductivity with the immediate surroundings of the alumi- 

 num disk. Thus it was hoped to make use of the latent heat of 

 freezing of the water, so that, in fact, the water jacket would act as a 

 constant temperature case, to prevent the cooling of the thermometer 

 below the freezing point of water. This worked excellently. 



A change was made from the practice of 1913 in attaching the 

 barometric element as a part of the pyrheliometer, instead of sending 

 up a separate meteorograph. Barometric elements, loaned by the 

 Weather Bureau, were mounted as shown at n, figure 5. The light 

 aluminum arm, 0, passing through a slot in the side of the cover 



