88 PETER GUTHRIE TAIT 



dryness of the air) and in consequence the mercury rises in the tube. In recent 

 experiments this rise of mercury has been carried to nearly 25 inches during dry 

 weather. But it can be carried much farther by artificially drying the air round the 

 bulb... I found, by inverting over the bulb of the instrument a large beaker lined with 

 moist filter paper, that the arrangement can be made extremely sensitive. The mercury 

 surface is seen to become flattened the moment the beaker is applied, and a few 

 minutes suffice to give a large descent, provided the section of the tube is small 

 compared with the surface of the ball. 



" I propose to employ the instrument in this peculiarly sensitive state for the 

 purpose of estimating the amount of moisture in the air, when there is considerable 

 humidity ; but in its old form when the air is dry. For this purpose the end of the 

 tube of the atmometer is to be connected, by a flexible tube, with a cylindrical glass 

 vessel, both containing mercury. When a determination is to be made in moist air 

 the cylindrical vessel is to be lowered till the difference of levels of the mercury 

 amounts to (say) 25 inches, and the diminution of this difference in a definite time 

 is to be carefully measured, the atmospheric temperature being observed. On the 

 other hand, if the air be dry, the difference of levels is to be made nil, or even 

 negative at starting, in order to promote evaporation." 



Experiments were made to test the applicability of the method ; but the 

 manipulation demanded more care and attention than could be expected from 

 a busy observer at a meteorological station. 



In 1887 Tait who had been for many years a keen devotee of the game 

 of golf was led to consider various physical problems suggested by the flight 

 of a golf ball, from the moment of impact of the club to its final fall to earth. 

 The first consideration is the manner in which the momentum of the club is 

 communicated to the ball. Given the club moving with a certain speed, with 

 what speed will the ball be projected ? This is the one stage over which the 

 player has any control. After the ball has left the club its further progress is 

 conditioned by the initial conditions of flight and the continuous subsequent 

 interplay between the moving ball and the surrounding air. 



Accordingly to study the laws of impact of various materials Tait set up a 

 simple but very effective form of apparatus which he humorously called the 

 "guillotine 1 ." The name occurs early in the third paragraph of the first paper 

 on Impact (Sci. Pap. No. LXXXVIII). The block whose impact on the 

 material was to be studied " slid freely between guide rails, precisely like the 

 axe of a guillotine." As this block fell and rose again after several rebounds, 

 a pointer attached to it bore with sufficient pressure upon the blackened 

 surface of a revolving plate-glass wheel. The curve traced out in this manner 



1 The name is preserved historically in the new Physical Laboratory of Edinburgh 

 University, the room in which the apparatus is now installed being called the Guillotine Room- 



