196 Proceedings of' the Royal Society 



crystals ; and 3. By mechanical abrasion. The crystals of the first 

 denomination he has examined are Brazil topaz, white fluor-spar, 

 hornblende, axinite, boracite, oxidulated iron, dodecahedral garnet, 

 diamond, and amethyst. The perfect crystals which he found best 

 fitted for presenting the phenomena he describes are alum, fluor- 

 spar, and calcareous spar. The facts related in this paper consist 

 of a description of a great variety of optical appearances, of which 

 it is impossible to give an abstract. 



2. Observations on the Pulsation of the Heart and Arteries. 

 By Dr Knox. 



The author here first describes briefly what has been done by 

 previous physiologists towards determining the difl^erences produced 

 on the pulse by varieties in posture, by digestion, by age, by stature, 

 by the period of the day, and by muscular motion. He then pro- 

 ceeds to relate the particulars of many new observations he has him- 

 self made upon these and other points connected with the circum- 

 stances which influence the rate of the heart's contractions. The 

 general results are, that in his opinion the pulse is more frequent 

 in the morning than at any other period of the day, abstracting the 

 effect of stimuli ; that the heart is more excitable at the same pe- 

 riod ; that the pulse is depressed by cold ; that it is excited by mus- 

 cular exertion, and more in the weak than in the strong, and also 

 more by exercise carried to fatigue than by moderate exercise ; that 

 it is most frequent in the young, and least frequent in the old ; and 

 that great varieties are produced by differences in posture, those 

 positions of the body occasioning the greatest excitement which re- 

 quire most muscular effort for their maintenance. 



February 20. — Dr Abekcromhy, V. P., in the Chair. The 

 following communication was read : 



I. On an Expression for the effort I'equired to Ascend Planes 

 of Different Inclinations. By Professor Forbes. 



The author states that, whilst it has been pretty generally agreed 

 by authors on animal mechanics that the measure of muscular 

 effort is generally expressed by the weight multiplied by the verti- 

 cal height through which it is raised, it is clear that this cannot be 

 universally true, since, in the particular case of horizontal motion, 

 no vertical height can be obtained ; and, when the ascent is verti- 

 cal, it is very unlikely that the same advantage of ascent should be 

 obtained as at lower angles. 



