COMMON THINGS THE ALMANACK. 



67. In almanacks generally the 3rd July and the llth August 

 are indicated as the first and last of the Doa DAYS. This com- 

 prises an interval of 40 days, which is generally the hottest part 

 of the summer. 



In the time of the ancient astronomers of Egypt and Greece, 

 the position of the equinoctial points and the tropics which deter- 

 mine the limits of the seasons was different from what it is at 

 present, and was such, that a remarkable star called Sirius, in 

 the constellation called CANIS MA JOE or the " great dog," rose in 

 the mornings immediately before the sun during the month of 

 July, of which it was considered the harbinger, and whose calorific 

 power was imagined to be increased by its influence. The idea 

 that this star, the Dogstar as it was called, exercised such an 

 influence, was no doubt countenanced by its extraordinary splen- 

 dour, being by far the most brilliant of the stars visible in the 

 northern hemisphere. The days, therefore, during which this 

 star ushered in the sun, and led, as it were, his way through the 

 heavens, were called CANICULAR DAYS or Doa DAYS. 



The prevalence of canine madness at this season may also have 

 had something to do with the name of dog days, or even with the 

 name of the constellation to which the star in question belongs. 



68. It might naturally be supposed that the days on which the 

 sun rises highest and remains longest above the horizon ought to 

 be the hottest, and that consequently the hottest interval of forty 

 days should be the forty days which comprise twenty before and 

 twenty after the summer solstice that is, from the 2nd June to 

 the 10th July. But this is just a month earlier than the interval 

 which is found by observation and experience to be on an average 

 of years the hottest part of the season. How then, it will be 

 asked, can this be explained ? 



That the calorific effect of the sun is greatest on the day of the 

 solstice is undoubtedly true ; but it is easy to show that the day 

 on which the sun imparts most heat is not the hottest day. 



To explain this, so far as it depends on the position of the sun 

 and the length of the days and nights, we are to consider the 

 following circumstances : 



As midsummer approaches, the gradual increase of the tempe- 

 rature of the weather has been explained thus : The days being 

 considerably longer than the nights, the quantity of heat imparted 

 by the sun during the day is greater than the quantity lost 

 during the night ; and the entire result during the twenty-four 

 hours gives an increase of heat. As this augmentation takes place 

 after each successive day and night, the general temperature con- 

 tinues to increase. On the 21st of June, when the day is longest, 

 and the night is shortest, and the sun rises highest, this augmen- 

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