142 THE ROYAL SOCIETY OF CANADA 
original erectors shows that the structure does not meet the require- 
ments of the process in question and condemns it for that reason as 
unsafe, or unless the bridge itself gives manifest signs of weakness and 
of pending collapse. Apart from such condemnation or manifest 
evidence of unsafety any traveller, not an expert engineer, refusing 
to use such a bridge, would thereby stamp himself as an unreasonable 
man. Moreover, an expert engineer, if he halted habitually at every 
bridge in his highway journeys in order to examine closely the structure 
before crossing or if he refused to make railway journeys because he 
could not halt the train at every bridge for a similar purpose, would 
thereby stamp himself as an equally, if not more, unreasonable man. 
The ordinary traveller has not the requisite knowledge; and the en- 
gineer, though he has the knowledge, could not with convenience or 
propriety make the necessary examination in the course of such journeys. 
For these reasons neither of them is in a position to judge of the matter 
for himself, and accordingly they are both required as reasonable men 
to use such bridges as safe if up to that time the contrary has not been 
specifically shown, 7.e., they are both required as reasonable men to 
accept the results obtained by the original, accredited operators of the 
above process as correct unless the contrary be shown. 
Equally effective illustrations of the above principle, with simpler 
processes but in quite as grave a connection, are to be found in the 
methods followed in preparing and supplying dangerous drugs for 
medical use. The manufacturing drug and supply houses, by errors 
in the preparation or labelling of drugs, might indeed introduce incor- 
rect, and possibly fatal, results into the druggist’s work; or the druggist 
himself, by an error in compounding a physician’s prescription, might 
produce such an incorrect and fatal result. But such results could 
come only through a deviation from the correct processes of preparing, 
labelling, and compounding drugs; for these processes, rightly followed 
by the employees of the manufacturing and supply houses and by the 
druggist, lead necessarily to the correct fillmg of prescriptions. And 
since reliable manufacturing and supply houses, by engaging such 
employees, thereby accredit them as operators of the correct process 
of preparing and labelling drugs, and the law, moreover, by licensing 
the druggist, thereby accredits him as an operator of the correct process 
of compounding drugs, the employees and the druggist are accredited 
operators of correct processes. Accordingly, those persons who are 
not in a position to judge of the matter for themselves are required as 
reasonable men to accept their results as correct, unless the contrary 
be shown; and in actual practice their results are, in point of fact, so 
accepted. Thus in actual practice the druggist, without further test, 
accepts and uses the drugs furnished him by the supply houses as 
