Street cleansing is effected either by hand-sweeping and hand-scraping, or by machinery. As to which is the most economical much depends upon the value of labour, and also upon the condition of the roads to be dealt with, but in point of time and as a general rule the value of a horse rotary brush-sweeping machine is undoubted, the only time at which such a machine fails to do effective work is on the occasions when the mud to be removed (owing to a peculiar condition of the atmosphere), has attained a semisolidity, and is of a stiff and sticky consistency, when it either adheres to and clogs the brushes of the machine, or is flattened by them on to the road instead of being removed.[165] The brushes of a machine last about 180 hours constant work, and then the old stocks can be easily refilled with bass at no great cost. The comparative work which can be done by a sweeping machine is about 11 to 1 of that effected by manual labour, so that the economy involved by the former method is evident. The strength and durability of the brooms used for the work of sweeping the streets is of some importance, as affecting the ultimate cost of the work, and some care and skill is required in their selection. Bass brooms are better than birch brooms for this purpose, and the bass of which the brooms are made should be sufficiently stout and of regular thickness; it should be tough and elastic, not old, dry, and brittle, each knot should be of uniform size and be firmly set, and the number of knots in each broom head is also a matter of choice. A convenient and fair test of the soundness of a broom is to soak it for a few days in water before issuing it to the sweeper, and then note the time it will last. The handles of the brooms should be made of alder wood. On the question of the extra work involved in street cleansing by its bad construction or by the materials of which it is constructed, climate must be considered, as well as the amount of traffic it has to bear, and also its gradient and the habits of the people residing in it.[166] The Superintendent of the Scavenging Department at Liverpool has made some observations and obtained some valuable information on these points, which he has detailed in a report he presented to the Health Committee of that borough in the year 1877, an abstract of which is as follows: GROSS COST FOR EACH TIME OF CLEANSING 10,000 YARDS SUPERFICIAL OF DIFFERENT DESCRIPTIONS OF ROADWAY IN THE BOROUGH OF LIVERPOOL.