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TRANSACTIONS OF THE 



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. 



This table of comparisons will give a good idea of the various currents 

 of air throughout the city. The Signal Office being near the river (within 

 one block), its minimum should be a little higher, while Captain Foster's 

 is about fifteen blocks, or over a mile, northeast of the Signal Office, and 

 on the corner of a wide street, while Mr. Gerrish's place of residence and 

 observation are very nearly one mile northeast of the Signal Office, and 

 are situated in the middle of the block. 



