59 



It is obvious that the delicate ivory point should be handled with 

 great care. 



135. The glass cylinder of the cistern should be washed in .soap 

 and water and thoroughly rinsed in copious applications of fresh 

 water. After this it should not he touched irith unprotected hands, 

 especially upon the inside. Wipe it thoroughly dry with a clean, dry 

 towel or handerchief, and polish with clean tissue paper. The 

 remaining wooden portions of the cistern should also be wnped 

 thoroughly clean and dry without touching the inside with the bare 

 fingers. Shake out of the bag as far as possible every little particle 

 of mercury that tends to remain in hidden corners and crevices. 

 These little particles are very apt to be dirty and impure, and should, 

 therefore, be removed. 



136. The several parts of the barometer should be replaced in the 

 following order : 



First, return the glass tube to its sheath, heing careful to place 

 the ivory point in the position in relation to the scale., or front of the 

 harometer^ formerly occupied ; also to avoid handling with the bare 

 fingers the end portion of the barometer tube where it dips into the 

 cistern, as a slight film of oil may be communicated to the mercury 

 of the cistern by this means. 



The glass cylinder, with its leather washers, one at each end, is 

 next placed in position, followed by the wooden piece i and the 

 metal flange ring R. The three long screws P are next to be in- 

 serted and partially screwed up. While these various pieces are 

 still loosely held by the screws, it is well to jostle the parts about 

 a little and twist the ring and boxwood pieces upon each other and 

 the glass cylinder. In other words, try to bring the surfaces in the 

 several joints nicely and uniformly into contact with each other, and 

 adjust the ring R so that the screws are not even imperceptibly 

 askew, but, when properly drawn up, produce a direct, uniformly 

 distributed pressure. Wlien the parts are thus adjusted the screws 

 P are to be tightened little by little, each one a little in turn after the 

 others, until all are drawn down together equally tight. The ob- 

 server must judge of this partly by the amount he has turned each 

 screw and partly by the resistance it offers to further turning. It 

 is not necessary that the screws be very tight, A judicious regard for 

 these ideas constitutes in part the skill of the expert and is the secret 

 of perfect joints. To disregard them produces leaky joints and 

 unequal pressures that are apt to break the fragile boxwood flanges 

 or crack the glass. 



Before describing the filling of the cistern, some tests and experi- 

 ments showing the purity and properties of mercury will be men- 

 tioned. 



137. Purity of mercury^ how tested. — Pure mercury is beautifully 

 brilliant and mobile, and does not exhibit the slightest adhesion to 

 clean, dry glass or porcelain surfaces, whereas the amalgamation of 

 the mercury with the slightest perceptible traces of foreign sub- 

 stances, such as lead, tin, zinc, etc., changes completely the character 

 of this peculiar substance. Each observer should try for himself the 

 following instructive and simple experiment : 



Prepare a small shallow, flat-bottomed porcelain cup or white piece 

 of chinaware, or glass vessel if the others are not to be had. Wash 

 and dry thoroughly without touching the inside with the bare fingers. 



