THE SUN'S HEAT. 15 



Tyndall has modified it by filling the upper vessel with mercury, 

 which is a better conductor of heat than water. 



For relative measurements, as for instance a comparison of the 

 amounts of heat received from the sun at different hours of the day, 

 Crova employs a slightly different instrument, of which Fig. 2, copied 

 from his paper in the " Annales de Chiraie " for February, 1880, is a 

 representation. 



An exceedingly sensitive alcohol thermometer, shown separately at 

 T, with a large bulb carefully blackened, is inclosed in a double-walled 

 sphere B, nickel-plated on the outside. An opening in the walls of 

 the sphere, carefully aligned with a similar opening in a double screen 

 E, allows a beam of light to fall upon the thermometer-bulb, the beam 

 being about two thirds the diameter of the bulb. The themometer is 

 constructed with a supplementary reservoir, r, at the lower end, by 

 means of which the end of the indicating column can be made to fall 

 near the middle of the scale at any temperature, the object being to 

 measure only changes of temperature, not absolute temperatures. The 

 bulb and tube are so proportioned that a degree on the scale is nearly 

 half an inch long, thus permitting great accuracy of reading. 



In order, however, to determine just how much heat is required to 

 raise the thermometer of this instrument 1, it is necessary to compare 

 it with one of the standard instruments, by exposing it to the sun at 

 the same time. 



This method of procedure, by which we determine the rate at 

 which a sunbeam of given dimensions communicates heat to a mea- 

 sured mass of matter, is known as the dynamic method ; it is some- 

 what inconvenient in requiring considerable time and a number of 

 readings. 



There is a different process for deducing the same results, which 

 has been employed by Waterston, Ericsson, Secchi, Violle, and others, 

 and may be called the statical method. It consists essentially in 

 observing how much the sun will raise the temperature of a body, 

 exposed to its rays, above that of the inclosure in which it is placed, 

 this inclosure being kept at a fixed and known temperature by the 

 circulation of water, or some such means. 



Instruments based on this principle are called actinometers. Of 

 these probably the most complete in its arrangements is that of Violle, 

 described in his paper upon the mean temperature of the sun's surface, 

 published in the "Annales de Chimie " in 1877. We give a diagram of 

 the instrument (Fig. 3). It consists of two concentric spheres of thin 

 metal ; the outer, twenty-three centimetres in diameter, the inner, fifteen 

 centimetres. The outer is polished on the outside, the inner is blackened 

 on the inside. The space between the two spheres is filled with water, 

 which is kept at a uniform temperature, either by mixing snow or ice 

 with it, or else by a current circulated through it by means of the 

 stopcocks tt. A sensitive thermometer, T, has its blackened bulb 



