STATE AGRICULTURAL SOCIETY. 283 



G Street, being on the north side of the street. The thermometer is exposed 

 to the full effect of radiation to the sky, there being no covering over it, and 

 is in the back yard subject to the full force and effects of the north winds, 

 which are cold in winter and hot in spring, summer, and fall. Mr. Ger- 

 rish's thermometer is about five or six feet above the ground. 



The Signal Service thermometer is a Green's standard minimum, self- 

 registering instrument, exposed on the roof of the Signal Office building, at 

 No. 117 J Street, sixty-one feet above the ground, and is in a single latticed 

 shelter, Signal Service pattern. The wind blows through the shelter in all 

 directions. 



The table below shows that during the thirty-one days of January there 

 were but two upon which each observer recorded the same minimum tem- 

 perature, and singularly, too, the minimum temperature was the sameon 

 both the days, being 34° on both the twelfth and nineteenth. The wind 

 was north, blowing nine miles per hour, and weather cloudy at 4 a. m. of 

 the twelfth, and southwest four miles per hour and weather cloudy on the 

 nineteenth. 



Captain Foster's record and the Signal Service record were the same on 

 eight days, as follows: Twelfth, 34°; thirteenth, 32°; seventeenth, 22°; nine- 

 teenth, 34°; twentieth, 38°; twenty-first, 41°; twenty-third, 49°; twenty- 

 fourth, 42°. There were but three days upon which the records of Mr. 

 Gerrish and the Signal Office coincided, those being the twelfth, 34°; nine- 

 teenth, 34°; and twenty-second, 45°. 



The average difference during the month was as follows: Captain Foster 

 1.8° lower than the Signal Service, 1.1° higher than Mr. Gerrish^ while the 

 records of the latter gentleman show an average difference of 2.9° less than 

 the Signal Service, and 1.1° less than Captain Foster. 



There were sixteen days in January that were cloudy at 4 a. m. The 

 average minimum temperature for those sixteen cloudy days at the above 

 hour, was: Foster, 39.9°; Gerrish, 39.8°; Signal Service, 41.6°— making the 

 latter only 1.7° higher than Foster, and 1.8° higher than Gerrish. _ The fif- 

 teen days that were clear or fair at 4 a. m., show an average minimum of 

 26.9° for Foster, 25.3° for Gerrish, and 29.5° for the Signal Service, making 

 the latter 2.6° higher than Foster, and 4.2° higher than Gerrish. The great- 

 est difference (6°) between the readings of Mr. Gerrish' s thermometer and 

 that of the Signal Service, occurred on the seventh, eighth, ninth, and six- 

 teenth. At each time the wind was from the north, and gentle in velocity, 

 and calm on the eighth. The weather was clear each day. 



The least difference was 1° — on the first, fourteenth, twentieth, twenty- 

 first, twenty-fourth, and twenty-sixth. The weather was rainy or cloudy 

 on each day, except on the fourteenth, when it was blowing briskly from 

 the north. There does not appear to be so much difference between the 

 readings of the Signal Office and Captain Foster's as there is between the 

 Signal Office and Mr. Gerrish's records. It appears from the above com- 

 parisons for January that, as a general thing, there is a difference of from 

 1° to 6° in clear weather between thermometers five feet above the ground, 

 and those located sixty-one feet above. The difference is greater in calm 

 weather, or during light to gentle winds, and when the temperature is be- 

 low the freezing point. The least difference in clear weather usually occurs 

 when it is quite windy — that is, when the wind is fresh to brisk and high. 

 In cloudy weather there is less difference than in clear weather, because 

 the clouds reflect the heat back to the earth, and prevent the rapid radia- 

 tion from all substances growing on the earth's surface, and prevents the 

 heat from escaping so rapidly from the earth's surface by radiation. 



