OF METEOKOLOGICAL INSTKUMENTS. 



ta 



PIG. 35. 



FIG. 42. 



FIG. 36. 



It is thus described by Mr. Southall: 



" The helio-pyrometer is an instrument which I have adopted for ascertaining as far as practicable the 

 heating power of the sun's unccncentrated rays. A self-registering maximum thermometer with black bulb, 

 made by CASELLA, on Professor Phillips's principle, is fixed on a cushion at the bottom of a box, the sides of 

 which are also cushioned, and a thick piece of plate-glass is laid upon the top to prevent currents of air 

 carrying off the heat, also with the view of preventing the cooling effects of terrestrial radiation. The box is 

 placed in such a position that the sun's rays may fall as nearly as possible perpendicularly on the glass, 

 and it may require a change of position two or three times in the day to accomplish this ; if, however, the sky 

 be free from clouds from n % to 12%, the maximum heat will be then obtained, and no change of position will 

 be required. A portion of the sun's heat, the amount of which may be calculated, is necessarily lost by 

 reflection frcm the two surfaces of the glass, but, as this amount bears an uniform proportion to the intensity 

 of the sun's rays, its loss is of no practical importance. A black bulb thermometer placed on grass, according 

 to the usual method, is much influenced by the cooling effects of evaporation from the grass and soil, and the 

 effect of the sun's direct rays is sometimes nearly lost by the counteracting power of strong currents of air, 

 and at all times the reading of the thermometer is lowered by a current which is generated by the heat of the 

 thermometer itself, as well as by terrestrial radiation. A small vessel has since been added, in which water 

 boils violently in the box, with a piece of tube to carry off the steam." 



Price, complete 



250 



33. Solar Intensity Apparatus, invented by Padre SECCHI, for measuring the 



comparative heat of the sun's rays. Two thermometers are here kept immersed 

 in a fiuid at any temperature, and a third surrounded by the same conditions, 

 but not immersed, is exposed to the rays of the sun. The increase of temperature 

 thus obtained is found to be the same irrespective of the temperature of the 

 fluid which surrounds it. Cylindrical form, about 3 X 10 inches. Price, 

 with the requisite three thermometers . . . 3 18 



Tripod stand, with universal joint, by which the above may be kept in any position 

 at any temperature . . . . . 17 6 



34. Actinomcter (Sir JOHN HEESCHEL'S), for ascertaining the absolute heating effect 



of the solar rays, time being considered one of the elements of observation. 

 To take an observation, the Actinometer is placed in the shade for one 

 minute and read off; it is then exposed for one minute to the sun's rays, 

 and its indication taken ; it is finally restored to the shade, and its reading 

 also taken, the mean of the two readings in the shade subtracted from that 

 in the sun, gives the actual amount of expansion of the liquid produced by 

 the sun's rays in one minute of time. (See Eeport of the Boyal Society on 

 Physics and Meteorology). . . . . . 5 18 



35. POUILLETT'S PYEHELIOMETEE (fig. 35), for ascertaining the effect of the sun's 



heat upon a given area by the number of degrees of heat imparted to mercury 

 in five minutes, as described in Dr. Tyndal's ' Heat Considered as a Mode 

 of Motion.' 440 



