LETTER TO CONST. HUYGENS. 20 may 1679 189 
80 sand-grains in the length of one 
80 inch. 
6400 sand-grains in a square inch. 
80 
p 512000 sand-grains in a cubic inch. 
27000000 animalcules which make 
3 up the bigness of a 
(Text-Fic. 1.] sand-grain. 
amounting to 1382400000000 animalcules in a cube 
inch, as ABCDEF. 
This number of animalcules is so great, that if one had 
as many sand-grains, of the bigness aforesaid, then one 
could lade with them more than 108 of our ordinary sand- 
lighters; that is, reckoning one schagt of sand (which is 
144 cubic feet) to every lighter. 
I have let my thoughts run likewise on the very 
little vessels that are in our bodies, and have judged that 
they are above a thousand times thinner than a hair of 
one’s head; and I have therefore put the proportion of 
the very little vessels thus in relation to the body, in 
order to arrive afterwards at the proportion of the 
vessels in the little animals. 
First of all, I sought to know how many hairbreadths 
are equal to the length of one inch; and having by me a 
copper rule, whereon the inches are divided into 3 parts, 
and each of these again into 10 parts (thus altogether, an 
inch divided into 30 parts), I laid hairs from my periwig 
upon these divisions; and observing them thus through 
a microscope, I judged that 20 hairbreadths are equal to 
3a of an inch. Consequently, there are 600 hairbreadths 
in the length of one inch. 
Further, I measured, roughly, the thickness of my 
body above the hips, and judged (taking one thing with 
another) that the diameter of my body was 8 inches.’ 
‘ If this were approximately correct, then L. was considerably more 
