PHOTOCiRAPHING LIC4HTNING WITH MOVING CAMERA. 127 



pressure precedino; and during storms. The instrument recommends 

 itself on account of the simplicity of its construction, its accuracy, 

 and the ease Avith which the readings can l)e made, less than one two- 

 hundredth part of an inch being easily read otf without the aid of a 

 vernier. 



The instrument shown in the illustration consists of a glass tube, 

 one-fourth inch inside diameter, 38 inches long, bent to an angle of 

 100° at a distance of 8^ inches from the closed end, filled with mer- 

 cury, and mounted with the open end dipping into a reservior, which 

 in this case is 1 inch in diameter. The open end can also l)e bent up 

 to form a siphon barometer. The scale is ])laced along the upper 

 ^,lanting part of the tube. A pointer is fastened to the lower part of 

 the instrument, the use of Avhich is to indicate when it is in an exact 

 vertical position, a mark on the wall indicating this position. The 

 surface tension of the mercury is overcome by gently swinging the 

 instrument and returning it to its vertical position. The calil)ration 

 may be done by comparison with a standard instrument or by actual 

 measurements. The bend of the tube may be made at a diiferent 

 angle, but should not be much less than 100° owing to the surface 

 tension of the mercury. With this angle the instrument will mag- 

 nify al)out six times, which has been found by the writer to be suffi- 

 cient for ordinary use. The range of the instrument described is 

 about 2 inches, and if a greater range is desired the slanting poi-tion 

 of the tube may be made longer or a diiferent angle of the bend be 

 substituted, although the proportions given have been found by 

 experience to give the best satisfaction. A tube with a small internal 

 diameter does not give good satisfaction owing to the surface tension 

 being greater. 



