1830.] 
On the Strength of Tied Beams . 
249 
the minute precision with which he examined every object. A striking illustra- 
tion of the fact that an object is frequently not seen, from not knowing how to 
see it , rather than from any defect in the organ of vision, occurred to me some 
years since, when on a visit to Slough. Conversing with Mr. Herschel, on the 
dark lines seen in the solar spectrum by Franenhofer, he inquired whether I 
had seen them ; and on my replying in the negative, and expressing a great desire 
to see them, he mentioned the extreme difficult} 7 he had had even with Franen- 
hofer’s description in his hand, and the long time which it had cost him in detect- 
ing them. My friend then added, I will prepare the apparatus, and put you in 
such a position that they shall be visible, and yet you shall look for them, and not 
find them : after which, while you remain in the same position, I will instruct you 
how to see them, and you shall see them, and not merely wonder you did not see 
them before, but you shall find it impossible to look at the spectrum without seeing 
them. 
On looking as I was directed, notwithstanding the previous warning, I did not 
see them ; and after some time I enquired how they might he seen, when the pre- 
diction of Mr. Herschel was completely fulfilled. 
It was this attention to minute phenomena which Dr. Woollaston applied with 
such powerful effect to chemistry. In the ordinary cases of precipitation the cloudi- 
ness is visible in a single drop as in a gallon of a solution, and in those cases where 
the cloudiness is so slight a-; to require a mass of fluid to render it visible, previ- 
ous evaporation, quickly performed on slips of window glass, rendered the solution 
more concentrated. 
The true value of this minute chemistry arises from its cheapness, and the ex- 
treme rapidity with which it can be accomplished : it may, in hands like those of 
Woollaston, be used for discovery, but not for measure. I have thought it more 
necessary to place this subject on what I consider ; ts true grounds, for two rea- 
sons. In the first place, I feel that injustice has been done to a distinguished phi- 
losopher, in attributing to some of his bodily, that excellence which I think is prov- 
ed to have depended on the admirable training of his, intellectual faculties. And 
in the next place, if I have established the fact, whilst it affords no better 
means of judging of such observations as lay claim to an accuracy more than 
human , it also opens to the patient inquirer into truth, a path by which lie may 
acquire powers that he would otherwise have thought were only the gift of nature 
to a favoured few. 
VI . — On the Strength of Tied Beams. 
Having, in the latter end of 1828, been led to make a few experiments on mo- 
dels of large beams, fitted with iron ties, with a view to ascertain the comparative 
economy and utility of the system in this country, for buildings of large dimen- 
sion, where the common trussed roof would probably be inconvenient and more 
costly, I am induced to offer you the results for publication in the Gleanings. 
It may be scarcely necessary to add, that this is hut a simple modification of the 
suspension principle with iron ; and that it has been successfully introduced in 
Great Britain, both for roofing, and in the construction of bridges. 
No. 1. A model of a saul beam, G| feet long, bearing distance feet, and l 
inch square, having been prepared, it was, in order to asceitain its deflections with 
different weights, first tried without any artificial tie j and it gave as follows, viz, 
lbs. oz. inches. 
2 11 0,1 
8 5 0,4 
14 2 0,65 
20 15 0,9 
26 12 1,175 
29 12 1,325 
The breaking weight of this specimen, unsupported, about 123 lbs. 
This specimen was taken down ; and a small cliaii ot about \ inch diameter fitted 
to it, by iron straps at the ends. Three supports cr small blocks of wood at about 
equal intervals werp applied between the chain ind beam, and the deflections 
noted as follows. 
lbs. inches. 
50 0,05 
102 ,2 
