THE STUDY OF TROPICAL SUNLIGHT. 23 



being least in green surfaces of vegetation and greatest in rocks 

 or red, clay soil such as is common in India under the name of 

 laterite. The actual number of hours of insolation per year on 

 the earth's surface, were the sky always clear, is greatest at the 

 equator and diminishes toward the poles, the ratio between 

 0° and 45^" being 1.83 to 1.34, although in the longer days in the 

 temperate zone the sunshine reaching the earth when the sun 

 is near sunrise or sunset is only a small proportion of that at 

 midday. As a result we have in the Tropics the added factor 

 of greater radiation from the earth's surface to augment the 

 direct influence of the sun, so that, as it has been shown above 

 that the influence of heat is the chief one to consider, this in- 

 crement due to radiation from the earth would be of decided 

 influence in the Tropics. In middle latitudes this factor has 

 been determined as about 0.1 of the solar insolation at midday, 

 but it acts during the entire twenty-four hours, whereas the sun 

 rises and sets.^* In northern climates the hours of insolation 

 during the short days are so few and the hours of radiation so 

 many during the night that the surface of the earth actually 

 steadily cools at certain times of the year, making one of the 

 factors which causes a winter season. 



Probably, untoward effects attributed to the tropical sun, if 

 any, are caused by the evenness of the climate rather than by the 

 differences of insolation at any one time; the absence of severe 

 contrasts, such as are given by the winters and the monotony 

 having their effect. However, Chamberlain ^^ investigated the 

 systolic blood pressure and pulse rate in 6,847 readings in 1,489 

 individuals of varying lengths of residence in the Philippines and 

 found that : 



"Reduced to the basis of a 12.5-centimeter armlet * * * the average 

 blood pressure for healthy white men in the Philippines (is) 115 millimeters 

 for those between 15 and 30 years of age and 118 millimeters for those from 

 30 to 40 years old. These figures are little if any below those to be expected 

 in a temperate climate when a 12.5-centimeter cuff is employed. * * * 

 There was no progressive tendency for the pressure to increase or to de- 

 crease with continued tropical residence up to a little over three years, 

 beyond which point our observations do not extend." This author also 

 found that "we may * * * conclude that the mean blood pressure for 



" Hann, Handbuch der Klimatologie (1910), 2, 23, calls attention 

 to measurements in Chinchochro, Laoango coast, near the equator. The 

 regular measurements of the surface of the earth exposed to the sun gave 

 temperatures generally over 75°, often 80°, and one time nearly 85° C. 



" This Journal, Sec. B (1911), 6, 437. 



