88 MR. D. WINSTANLEY ON THE RADIOGRAPH. 



shall make a tracing of its curves on a cylinder driven by 

 clockwork at an even speedy and the " radiograph '' is 

 complete. 



Concerning the actual instrument I use^ its wheel is 

 "'3 inches in diameter, and the weight thereof a trifle 

 more than two pounds. The other portions of the appa- 

 ratus are of the same dimensional proportions as are indi- 

 cated in the sketch. Of course some delicate method of 

 recording has to be employed, and I have thus far used 

 the smoked-paper process so much adopted in the obser- 

 vatories of France. In this way the " radiograms " which 

 illustrate this paper were obtained. 



When using the instrument to record the radiance of 

 the sun, I have hitherto exposed it in a box of copper sur- 

 mounted by a dome of glass into which the bulbs of the 

 thermometer project. The line which joins them is in the 

 plane of the meridian of the place and the black bulb to 

 the north. The box itself is supported, at an elevation of 

 four feet or thereabouts, upon a stand of wood, the legs of 

 which are fimly embedded in the ground. The stand 

 itself is located at the extremity of a garden which over- 

 looks a valley and the sea. A small window in the box 

 permits the movements of the train to be seen and the 

 promptness with which the apparatus acts to be observed. 

 If a cloud " no bigger than a man^s hand,^^ and " light 

 as a feather" in its texture, floats before the sun, and 

 occupies but three or four seconds in its transit, its pre- 

 sence, the duration of its passage, and the degree of 

 thermal obscuration it eff'ects are at once set down. 



The cylinder of the radiograph passes over a space of 

 •875 of an inch per hour, a somewhat open range; but, as 

 will be seen on reference to the tracings, the needle often 

 moves for some considerable distance in both directions 

 along the same thin line, thereby showing a practical 



