220 Proceedings of Royal Society of Edinburgh. [sess. 
differ in duration, in suddenness of rise and fall, in the degree of 
their irregularity of vibration, and in the average period of those 
vibrations. This average period may also increase or decrease 
during the utterance of a given sound, and may do so more or less 
rapidly. These distinctions are all apprehensible by the ear, and 
they are vital to the recognition of consonants. The function of a 
vowel is to constitute the strong sound which forms the climax of 
a syllable. The function of a consonant is to connect those stronger 
sounds together, so as to form waves of sonority, i.e ., syllables. 
Every syllable must have a vowel, hut it need not have a conso- 
nant ; a vowel by itself is a wave of sonority, because it always 
has a rise and a fall of intensity, however brief and feeble. 
Consonants serve, however, both to make the minima of sonority 
more marked, and at the same time to enable us to multiply 
enormously the number of significant syllables at our command, 
by changing the consonants. 
The essence of a consonant, then, is simply to he less sonorous 
than the adjacent vowel ; and the essence of a vowel is to he more 
sonorous than the adjacent consonants. Some speech-sounds are 
so sonorous as to he practically always used as vowels ; others are 
so wanting in sonority as to he practically always used as conso- 
nants. But there are some of a middle kind, which can he used 
either way. When this is to he done, their relative sonority is in 
practice heightened or reduced by an addition or subtraction of 
stress, i.e.y of lung-pressure. In this way it is easy to pronounce 
aia or aua so as to make both i and u function distinctly as conso- 
nants. (I use the letters here and elsewhere in their Latin values, 
except in spelling English words.) But the s used in hissing, the 
sh used in hushing , the Z in bottle , the n in eaten (eat’n) each 
function distinctly as vowels, because they constitute, or have the 
power to constitute, each by itself, a syllable. 
But some syllables begin or end with more than one consonant. 
When syllables are built in this way, inherent differences of 
sonority again obtrude themselves. The syllable is essentially a 
wave of sonority, one rise and one fall. Therefore consonants 
cannot he built into it at random, hut only in the order of a rising 
and falling sonority. A syllable can no more begin with It or nt 
than it can end with tl or tn. This is simply because Z and n are 
