45, CORNHILL, E.G., AND 122, REGENT STREET, W., LONDON. 



FIG. 42. 



scale is engine-divided on the stem, and the divisions protected by a glass 

 shield. In use, it should be placed horizontally, with its bulb in the full 

 rays of the sun, resting on grass, and, if possible, so that lateral winds should 

 not strike the bulb. The directions for use are identical with those for the 

 determining of the temperature of the air. Fig. No. 41. Price, 1 10 



50. Vacuum Solar Radiation Thermometer (fig. 42). This instrument 

 consists of Negretti and Zambra's blackened bulb Radiation Thermometer, 

 enclosed in a glass tube and globe, from which all air is exhausted, as suggested 

 by Sir John Herschel in the Admiralty Manual of Scientific Enquiry, in 1849. 

 Thus protected from the loss of heat which would ensue if the bulb were 

 exposed, its indications are from 20 to 30 higher than when placed side by 

 side with a similar instrument with the bulb exposed to the passing air. At 

 times when the air has been in rapid motion, the difference between the reading 

 of a thermometer giving the true temperature of the air in the shade, and an 

 ordinary solar radiation thermometer, has been 20 only, whilst the difference 

 between the air temperature and the reading of a radiation thermometer in 

 vacuo has been as large as 50. It is also found that the readings are almost 

 identical at distances from the earth varying from six inches to eighteen inches. 

 By the use of this improved Solar Radiator the amounts of solar radiation at 

 different places are rendered comparable ; with the exposed bulb Thermometer, 

 (fig. 41) the results could not be compared, as the bulbs of the thermometers 

 would be under very different conditions as to exposure and currents of air. 

 This new arrangement gives the readings very much more uniform, and is 

 found to be a decided improvement. Price, 150 



Instructions for use same as No. 48. 



FIG. 43, 



