METEOROLOGY AND ALLIED SUBJECTS. 249 



the given location — that is to say, the dififereuce in the readings of tlie 

 upper and lower ends — can be indicated by (*, Jc). If, now, we neglect 

 the corrections of the short intervals that lie between tbe ends of the 

 thread and the principal points between i, Jc, then we have the rela- 

 tioD /i.k equals Ai — A k -\- {i, Jc). If we now shove the thread so 

 that its ends come in the neighborhood of other principal points, then 

 we have new equations in which the left-hand side is the same, but on 

 the right-hand side in place oft Jcwe have successively i—1, Jc—1 ; t— 3, 

 1c— 2. If, now, we subtract each equation from the ones followiug it, then 

 we have a new series of equations that may be represented by '9i— '9t = 

 (/— 1, Jc—1.) — (i, Jc). The sum of all the left-hand side of this equation 

 must disappear. {Z. 0. G. 21., XIV, p. 426.) 



Pernet has investigated a method of computing the variations of freez- 

 ing points of thermometers. He says that the case often occurs that no 

 ice is conveniently at hand for the determination of the freezing point 

 before and after the measurement has been made of some high tem- 

 perature; but even then oue can determine the variation of the freezing 

 point with a degree of accuracy that is generally sufficient, provided 

 that we know the freezing point as affected by the change from zero to 

 100° centigrade, and the temporary freezing point — that is to say, the 

 freezing point corrected for the depression depending on the time — 

 that is to say, on the exposure of the thermometer for a long time to the 

 temperature of the room, and which depression is nsuallj' proportional 

 to the time. This last condition is generally' fulfilled for thermometers 

 that are more than six mouths old, or even in a shorter time if the ther- 

 mometer has been slowly cooled down after its determination of the 

 boiling point. Let r be the reading of the thermometer corrected for 

 the caliber and the value of the degrees; let c be the temporary de- 

 pression of the freezing point and •/ be the maximum depression of the 

 freezing point for the range from zero to 100° C; then, after a long 

 warming at the temperature r, we have the true temjjerature given by 

 the following formula : 



1002 



^ — ' ^+ TtU\2 



If, now, we pass directly from these comparisons to such as are made at 

 steadily increasing temperatures tj t2 without allowing the thermome- 

 ter to cool down in the mean time, then this formula holds good for the 

 higher temperatures, and we simply substitute rj, t2, «&c., for r. But this 

 formula does not hold good if between two series of observations the ther- 

 mometer has ever been exposed to low temperatures. In this case a new 

 temporary freezing point Ci must be used as the starting point. {Z. 0. 

 G. M., Vol. XIV, p. 206.) 



Winstanley has given his radiograph a form convenient for continu- 

 ous self records. The instrument consists essentially of au air ther- 

 mometer, having its bulbs bright and black, respectively. The tube 

 connecting these is bent around a brass circle, so that the bulbs or res- 



