4 o PRtNCtPLES OF GENERAL PHYSIOLOGY 



The experimenter, who uses a slide rule or a table of logarithms to diminish his arith- 

 metical labours, should often feel grateful to the inventor of logarithms. This was Napier 

 of Merchiston, whose portrait will be found in Fig. 2ti. Merchiston Tower is seen in Fig. '_'T. 

 For most purposes, the short straight form of slide rule gives sufficient accuracy. If a greater 

 number of significant places is required in the result, the spiral form of Fuller is very 

 convenient in use. It is made by Stanley. The Handbook to the Exhibition at the 

 Tercentenary of Napier, published by the Royal Society of Edinburgh in 1914, will be found 

 useful in connection with the history and use of logarithms, as well as with other aids to 

 calculation. 



There is sometimes an unfounded prejudice against smoothed curves, but, if the data show 

 any sort of regularity, the course of the phenomenon is more accurately shown by such a 

 curve, since it eliminates accidental errors. It may be useful to describe the method, slightly 

 mollified from that of Ostwald -Luther (1910, pp. 28-3(1), which I have found the mo-i 

 convenient one for drawing curves for reproduction. The experimental values are first 

 marked by -f at the intersection of the co-ordinates, given appropriate values, on squared 

 paper; a curve is drawn as smoothly as possible by hand, using a pencil, through these 



FIG. 27. MKRCIIISTON TOWKK. 



(Published by S. Hooper, 1790. 

 Iferchistmiam, 191-2-13.) 



From The 



points. The paper is pasted on to a piece of moderately thick cardboard, which is then cut 

 with scissors along the curve, so as to obtain a template. The movement of the hand in this 

 operation is very regular, being sensitive to the least deviation from a regular course. 

 Ostwald states that the co-ordination of hand and eye is sensitive to the second, or even 

 third, differential coefficient. This template is used to draw a curve in pencil on Bristol 

 l>oard, which curve is then inked in by means of a French curve or a flexible curve. (The best 

 the " J. FL. B." made by Harling, Finsbury Pavement.) It will be plain that the lai^-r 

 the scale, within limits, to which the curve is drawn, the better it will look when reduced 

 for publication ; the slight inaccuracies in the use of the French curve will be invisible. The 

 little work by Howard Duncan on "Practical Curve Tracing" (Longmans) will be useful. 



A word of caution may be allowed. Although an equation may express in 

 one line what would require pages of verbal description, it must not b 

 forgotten that it is, after all, but a kind of shorthand, and must never be 

 permitted to serve in place of a clear conception of the process itself. The 

 same thing may be said of structural formulae in chemistry, which are only a 

 very convenient way of expressing certain facts in the play of molecular forces, 

 whose nature is as yet unknown. This fact sometimes seems to be in danger 

 of being forgotten, and "bonds" regarded as actual material threads holding 

 alums together. 



