EXPERIENCES 67 



the regular return of such tins, tubs, or barrels, and 

 allowance given for them in the bills. Or such barrels 

 may be bought outright, and used over and over 

 again. Usually, however, oil merchants charge for 

 the hire of the tins, and deduct the price of the hire 

 if they are returned uninjured. It is the duty of the 

 foremen to see that they are carefully handled. The 

 amount of oil required per day should be supplied 

 only on the written voucher signed by the head 

 of the department organising the campaign. The 

 vouchers should be returned with the monthly ac- 

 counts, and they serve as a check on the bills. The 

 foremen should give a receipt every day to the oil 

 purveyor, stating that the amount received is as 

 ordered, and that its condition is satisfactory for 

 sometimes it is so mixed with water by the middleman 

 that it is useless as a mosquito larvicide. The oil 

 refined and crude may be supplied already mixed 

 by the purveyor, or it may be mixed by the men of 

 the brigades as convenient or cheapest. In some 

 countries the percentage of refined oil to crude oil is 

 found, by experiment as to its larvicide properties, 

 to vary considerably according to the temperature 

 and humidity of the air ; then allowance must be 

 made. If the oil is too thin, more crude petroleum 

 must be added ; or if too thick to spread evenly, more 

 refined oil than equal parts will be required. 



Each mosquito brigade will need an oil-barrel 

 fitted to a pair of wheels with hand shafts so that it 

 may be pushed through the streets ; horse transport 

 is an unnecessary expense. The barrel, which may 



